kanzensitunaigai ni ha dassoubousi ga hituyou desu.

Because 必要です often uses the pattern:

X には Y が必要です
= For X, Y is necessary

In this sentence:

• 完全室内飼いには = for strictly indoor keeping
• 脱走防止が必要です = escape prevention is necessary

The に marks the thing that the necessity applies to, and は turns that whole part into the topic.

So 完全室内飼いには means something like:

• when it comes to strictly indoor keeping
• for keeping a pet entirely indoors

If you used only は, it would sound less natural with 必要 in this structure.

Because 必要 normally marks the thing that is needed with が.

So:

• 脱走防止が必要です = escape prevention is necessary
• literally, escape prevention is the thing that is needed

This is a very common pattern:

• お金が必要です = Money is necessary.
• 時間が必要です = Time is necessary.
• 準備が必要です = Preparation is necessary.

So in your sentence, 脱走防止 is the specific thing being identified as necessary.

完全室内飼い is a noun phrase meaning keeping an animal entirely indoors.

It is made of:

• 完全 = complete / entirely
• 室内 = indoors
• 飼い = keeping / raising (an animal)

So the whole phrase means something like:

• strictly indoor keeping
• keeping a pet exclusively indoors

This is especially common when talking about cats and dogs.

Because here the word is being used as a noun, not as a plain verb.

• 飼う = to keep / to raise / to own (an animal)
• 飼い = keeping / raising

Japanese often turns the ます-stem of a verb into a noun-like form.
For 飼う, the stem is 飼い.

So:

• 室内で飼う = to keep indoors
• 室内飼い = indoor keeping / an indoor-raised pet

This kind of noun formation is very common in Japanese.

Yes, it functions as one noun phrase.

Japanese often combines several nouns into one compound:

• 完全
• 室内
• 飼い

Together they form a single concept.

In normal Japanese writing, you would usually see the sentence without spaces:

完全室内飼いには脱走防止が必要です。

The spaces in learning materials are often just there to help learners see the parts.

室内飼い already means keeping an animal indoors, but 完全 makes it stronger:

• 室内飼い = indoor keeping
• 完全室内飼い = completely / strictly indoor keeping

So 完全 emphasizes that the animal is not supposed to go outside at all.

This matters because the sentence is about preventing escape, which is especially important if the pet is meant to live strictly indoors.

It is a compound noun:

• 脱走 = escape / getting loose
• 防止 = prevention

So 脱走防止 literally means escape prevention.

In more natural English, depending on context, it could mean:

• preventing escape
• escape prevention measures
• making sure the pet cannot get out

It is a very noun-heavy, compact Japanese expression.

It could. Japanese often has two ways to say similar things:

1. noun style
   脱走防止が必要です
   = escape prevention is necessary

2. verb style
   脱走を防ぐ必要があります
   = it is necessary to prevent escape

Both are natural. The version in your sentence is more compact and formal-sounding because it uses noun compounds.

Japanese often prefers this kind of compressed noun style in explanations, rules, notices, and advice.

必要 is usually treated as a na-adjective or an adjectival noun.

That means it can work like this:

• 必要です = is necessary
• 必要なもの = a necessary thing
• 必要があります = there is a need

In your sentence, 必要です means is necessary.

So the structure is:

• 脱走防止が必要です
• escape prevention is necessary

Yes, very possibly.

Japanese often leaves out things that are understood from context.
This sentence sounds like part of a discussion about pets, especially cats.

So the full idea might be something like:

• For keeping a cat completely indoors, escape prevention is necessary.
• If you keep pets strictly indoors, you need measures to stop them from escaping.

But Japanese often does not mention the animal if the topic is already known.

It can feel like either one in English, depending on context.

Literally, it refers to the practice/style of keeping an animal entirely indoors.
But in natural English, that may become:

• for indoor-only pets
• for keeping a pet strictly indoors

Japanese noun compounds often leave this kind of wording flexible.

So even though the Japanese expression is compact, the natural English version may need to unpack it a little.

Yes, it is fairly neutral-to-formal.

Reasons:

• it uses compact noun compounds like 完全室内飼い and 脱走防止
• it ends with です
• it sounds like informational or advisory writing

This is the kind of sentence you might see in:

• a pet care article
• a website
• a vet’s advice page
• a leaflet about cat safety

It is not stiffly formal, but it is definitely more written/explanatory than casual conversation.

Yes, in natural English that can work well.

Even though the Japanese literally says:

• escape prevention is necessary

natural English may prefer:

• you need to prevent escape
• you need escape-prevention measures
• it’s necessary to make sure the pet can’t get out

So a more literal translation helps you understand the grammar, but a more natural translation may sound better in English.

Yes, basically. 飼う is used for keeping/raising animals.

So 室内飼い is specifically about pets or animals being kept indoors.

You would not use 飼う for people.
For children, for example, completely different verbs are used.

So this word strongly tells you the sentence is about animals.

A helpful breakdown is:

完全室内飼い には 脱走防止 が 必要 です。

• 完全室内飼い = strict indoor keeping
• には = for / when it comes to
• 脱走防止 = escape prevention
• が = marks what is necessary
• 必要です = is necessary

So the grammar pattern is:

X には Y が必要です
= For X, Y is necessary

That is the key pattern to learn from this sentence.