someone else's problem; fire on the opposite shore
Idiomatic expression meaning a matter that does not concern oneself; literally 'fire on the opposite shore'. Often used when someone is indifferent to a problem because it doesn't affect them.
To him it was someone else's problem, but for us it's a big issue.
隣の部署のトラブルを対岸の火事と思っていたら、自分の部署にも影響が出た。
I thought the trouble in the neighboring department was none of my business, but it ended up affecting my own department.
Also means 'someone else's affair', but 対岸の火事 emphasizes the physical or emotional distance, like a fire across the river that doesn't threaten you.
From the literal image of a fire on the far bank of a river, which poses no danger to oneself. The phrase is used metaphorically to describe indifference to others' troubles.