An engaging device is provided which is capable of advantageous switchover from an engaging position to an open position. Also provided is a transfer instrument employing the structure of such an engaging device. The transfer instrument (A) includes the engaging device (X) which is capable of gradually moving an engaging surface (23) widthwise of an engaging surface (13) to retract two engaging components relative to each other by pivotally moving a dangling wall (11) of a transfer instrument body (1) and a sidewall (21) of an arm (2) relative to each other to press a sliding contact edge (24) against a guide edge (14) gradually from a first end portion (14a) toward an opposite second end portion (14b) of the guide edge (14).