To provide a locking device which can surely and strictly perform locking in a simple constitution and be unlocked only by a permitted person by removably providing a key cover for refusing an access from the outside in a keyhole provided in the locking device main body.

A receiving member 3 is fixed to the upper part of the outer periphery of a keyhole 1a, a key cover A is fitted toward the keyhole 1a, and a pin 5b is engaged with an engagement hole 3b of a cap receiving member 3 to lock the key cover A in the keyhole 1a. An unlock signal transmitted from an external R/W terminal 10 is received by a control chip 4 built in the fit-in part 2a, and according to a control signal sent from the control chip 4, the pin 5b is drawn in and disengaged from the engagement hole 3b to enable access to the keyhole 1a.