A testing device (1) with a test button (4) and a rod-shaped plunger (5) for testing a locking device that can be actuated on a door (2) pivotable around a hinge on a container (3) is to be improved in terms of simple design and mounting. To simplify design and mounting, a fastening structure (8, 7) for the movable, rotatable mounting of the testing device (1) is arranged on the door (2). The end (13) of the plunger pointing toward the container (3) is provided with a holding structure (14) which is introduced into a mounting support (16) arranged permanently on the container (3) such that the holding structure (14) is out of engagement with the mounting support (16) in normal operation, and in testing operation. The holding structure engages the mounting support (16) in such a way as to limit the pivoting stroke of the door (2).