To prevent a key from slipping out at a position other than an insertion position of the key by making the width of an end of a plate tumbler smaller than the width of a groove at a position corresponding to an outer cylinder and making the width of the end of the tumbler larger than the width of a groove located at any other position.

A plurality of plate tumblers 3 are disposed, spaced from one another, in a rotor 2 which is rotatably disposed along the inner wall surface of an outer cylinder 1. The tumblers each are urged in one direction by a spring 4 and are movable in a direction normal to the axis of the rotor. The relationship between the width of both ends 3c, 3d of the tumbler 3 and the length of grooves 1c, 1d of the cylinder 1 is W 31<W 11<W 32<W 12. When the rotor is turned through 180 degrees, W 11<W 32, and hence the tumbler 3 is not put into the groove 1c so that a key can not be drawn out. It is preferable to provide such a mechanism for preventing a key from being drawn out for at least two tumblers or desirably for all tumblers. Consequently, inexpensive tumbler locks can be obtained.