To make smaller a dimension of an insertion direction of a key than that of the conventional cylinder lock and, at the same time, to promote safety of a cup lock by incorporating a variable tumbler device into the cup lock.

A cup lock is constituted of a fixed inside cylinder 1 formed in the shape of a bottomed cylinder, an outside cylinder 2 loaded to the circumference of the inside cylinder in a rotatable manner, a key guide 3 provided to the front plate 11 of the inside cylinder 11 in a rotatable manner, a plurality of tumblers 6 detachable to the outside cylinder 2 and a clutch 8 or the like provided to the bottom plate 21 of the outside cylinder 2 so as to corresponding to a plate-like key inserted into the key guide 3, and operation making both ends 6a and 6b of the tumblers 6 coincide with a shear line S can be accomplished by the rotation of the key. A constrained member 3 interlocking the selected tumblers 6 with a non-operable position is provided.