To quickly inspect a bonded condition after lap connection of waterproof rubber sheets.

In this inspecting method, high-pressure air is fed into a thermally fused lap part of a waterproof rubber sheet lap-connecting part constituted by sandwiching a thermally fusible adhesive film 6 between terminal lap parts of connected waterproof rubber sheets 1, 2 to be thermally fused via an air injection needle, and then the bonded condition of the waterproof rubber sheet lap connecting part is inspected, based on the level of pressure reduction of the injected high pressure air after the lapse of a fixed testing time.