A pit cover open/close device includes a closing limiting stopper for a pit cover which is disposed in a position corresponding to a top of a roller of a chassis dynamometer, guides disposed on a stationary side of the pit in an opposed relation to both side faces of the roller of the chassis dynamometer and extending along a circumferential direction of the roller of the chassis dynamometer, a two-part split pit cover supported by the guides and including two pit cover parts into which the pit cover is split from a portion thereof corresponding to the position of the stopper, the two pit cover parts being movable in an opening direction and a closing direction, respectively, and a cylinder including a movable rod that is connected to one of the two pit cover parts and a body that is connected to the other of the two pit cover parts, the cylinder being driven to allow the two pit cover parts to move along the guides in the opening direction and the closing direction.