To provide a disk cylinder lock which can make it difficult to unlock by a picking operation and can prevent the problem of frequent lock-picking crime in conventional disk cylinder locks.

The disk cylinder lock is used for locking of the following mechanism. Triangular tooth-like internal gear ratchet wheel shapes are formed on the inner side of an external cylinder (1) to provide a number of triangular grooves parallel to a central axis of the external cylinder (1). These grooves are provided as inner grooves (13) of the external cylinder when the unlock direction is clockwise. When the unlock direction is anticlockwise, the grooves are provided as inner grooves (15) of the external cylinder. When the unlock direction is clockwise, a disk (4) having an end (14) with a shape that matches the shape of the inner grooves (13) of the external cylinder is provided in an internal cylinder (2). Meanwhile, when the unlock direction is anticlockwise, a disk (4) having an end (16) with a shape that matches the shape of the inner grooves (15) of the external cylinder is provided in an internal cylinder (2).