To provide a protection device for a cylinder lock which can reduce production cost while maintaining crime prevention effects to some extent.

The cylinder lock protective device comprises: a housing 1 provided in the upper part of a key hole Ra of a cylinder lock; a shutter 2 which is movable between a closed position and an open position by being rotated in the housing 1, so as to open and close the key hole Ra; a moderation means 3 which gives moderation to the shutter 2 when the shutter 2 is in the open position and in the closed position; and an operating means 6 which can move the shutter 2 between the open position and the closed position, by fitting into a fitting recess 2a formed in a predetermined site of the shutter 2 and rotating, and in which the shutter 2 is rotatable together with the operation means 6 after moderation by moderation means 3 is given. Regulation of rotation of the shutter 2 is exclusively attained by the moderation means 3.