http://www.kokuminhogo.go.jp/en/pdf/missiles_flying_0921.pdf
It may take less than 10 minutes that missiles reach Japan after being launched, If missiles are about to hit Japan possibly, J-ALERT system provides emergency information through various devices such as sirens and emaills.
1.Evacuate immediately
2.Collect precise information immediately.
Follow official instruction and take action calmly.
When you recieve a message,
Stay calm and take action immediately.Outside
Go into a nearby building or underground,
"a sturdy building if possile.
such as underground shopping arcade and subway stations.
Can't find building nearby？
Lie flat on the ground or take cover
behind anything that might offer protection.
Inside
Stay inside well away from window
or stay in a windowless room.
J-ALERT Message【Example】+announced ONLY in Japanese
直（ただ）ちに避難（ひなん）。直ちに避難。直ちに建物（たてもの）の中（なか）、又は地下（ちか）に避難して下さい。ミサイルが落下（らっか）する可能性（かのうせい）があります。直ちに避難して下さい。
Evacuate immediately. Go into a building or underground nearby. A missile may reach Japan. Go inside immediately.
ここ、「又マタ」と「下クダ」の二つの漢字に振り仮名振ってないのは、
恐慌のあまりの所業なんだろうかと想像しました。ほかはルビ振ってるのに。
A missile hit in nearby area？
Outside: Cover your nose and mouth with cloth. Evacuate from an impact area. Evacuate toward the windward side or into sealed room.
Inside: Stop a ventilator, close windows and seal of a room.
Look in advanxe. Civil Protection Portal Site (English)
