What it means
ではないか is used at the end of a statement to seek agreement with an emphatic conclusion. It often means “surely ..., isn’t it?” or “... right?” rather than asking a completely open question. The speaker usually believes the statement is true and expects the listener to recognize or confirm it. With verbs and i-adjectives, の is normally inserted before ではないか.
Usage note
The familiar form is ではないか. The conventional formal or polite counterpart is ではありませんか; ではないですか is also used as a colloquial polite form, but it is not simply the standard polite equivalent of ではないか.
How to use it
これは図書館ではありませんか。
This is surely the library, isn't it?
この方法は便利ではありませんか。
This method is surely convenient, isn't it?
この道は長いのではありませんか。
This road is surely long, isn't it?
田中さんは明日来るのではありませんか。
Tanaka is surely coming tomorrow, isn't he?
これは図書館ではないか。
This is surely the library, isn't it?
この方法は便利ではないか。
This method is surely convenient, isn't it?
この道は長いのではないか。
This road is surely long, isn't it?
田中さんは明日来るのではないか。
Tanaka is surely coming tomorrow, isn't he?
Example sentences
この地図は駅までの近道ではないか。
Isn't this map a shortcut to the station?
明日のテストはそれほど難しくないのではないか。
Tomorrow's test surely isn't that difficult, right?
この方法なら、時間を節約できるのではないか。
If we use this method, we can save time, can't we?
この図書館は町で一番静かな場所ではないか。
Isn't this library the quietest place in town?