To prevent wrong unlocking, by a simple structure wherein a key and rotor hole of a cylinder lock equipped with plate tumblers is formed to have a round section and a rod-shaped key having ridges is used for unlocking so that a flat matter such as scissors can not be inserted.

A key 1 is formed by providing a guide groove 1b and also a plurality of ridges 1a in a rod-shaped part 1e having a round section and by fitting a knob 1c of resin or the like to the end part thereof. This key 1 is inserted into a keyhole 3a of a rotor 3 of a cylinder lock 5 used for an ignition switch or the like, with the guide groove 1b of the key 1 made to engage with a projection 3b, and plate tumblers 2 are disengaged from recessions 5a for engagement, so that the rotor 3 be made rotatable. The structure of the cylinder lock 5 is the same as that of an existing plate-shaped key, except the keyhole 3a, and the lock can be manufactured at a low cost. A flat matter such as scissors can not be inserted into the keyhole 3a. Accordingly, it is possible to prevent wrong unlocking and to enhance a burglarproof effect.