To provide a protective cap of a lock which is safe without protruding a cap cover to a near side when opening, and is new, novel, and simple structurally and in terms of design.

A protective cap C makes a cap cover 20 engaged with and connected to a cap base in a slidable manner between a closed position blocking a key operation hole 16 by covering a cap base 15 and an open position opening the key operation hole 16. The protective cap C is always energized to the closed position by energizing springs 45 and 45 and is pressed against stopper pins 40 and 40 when it is in the open position to be positioned and held at the open position. When it is in the closed position, the cap cover is slid against the energizing springs 45 and 45 to the closed position to fully cover the cap base so as to attract a lock pin 35 by a magnet M and engage a shaft portion 35a to a hooking hole 60. The cap cover is hooked to the cap base to be automatically locked.