To provide a shutter capable of completely preventing dirt, dust, or small bugs from invading into the inner tube of a cylinder lock.

A shutter storage part 17 is provided recessedly at the rear of a key guide 5, and a shutter 18 extending in the lateral direction of a keyhole 7 and formed of a platy magnetic material having those dimensions sufficient to cover the keyhole 7 opening in the shutter storage part 17 is disposed in the shutter storage part 17. Also, the shutter 18 is guided so as to be rotated about a rotating shaft 21 provided closely to the keyhole 7 and passed through the outside of the keyhole in lateral direction, and the shutter 18 for covering the keyhole 7 located at a close angle position is attracted by a permanent magnet.