It's high time he arrived.
彼はもう着いている［いていい］はずだ
It's his deal.
彼が親だ
It's hot in here.
この中は暑い
It's idle to think you can change the world by yourself.
自分１人の力で世界を変えようと思ってもむなしいことだ
It's just 3 o'clock.
３時きっかり
It's just a friendship thing.
（男女が恋愛関係ではなく）単なる友人（関係）だ
It's just a joke.
ほんの冗談だよ
It's just a phase he's going through.
((略式))そういう年ごろなんだ
It's just a thought.
((略式))１つの考えとして言ってみただけだけど
It's just like him to do something like this.
こんなことするとはいかにもあいつらしい
