kanzenshitsunaigai ni wa dassouboushi ga hitsuyou desu.

Indoor-only care requires escape prevention.

Breakdown of kanzenshitsunaigai ni wa dassouboushi ga hitsuyou desu.

• は: topic particle
• です: to be
• が: subject particle
• 必要ひつよう: necessary

Questions & Answers about kanzenshitsunaigai ni wa dassouboushi ga hitsuyou desu.

Why is it に は here? Why not just は?

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.

---

Why is が used after 脱走防止?

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.

---