PURPOSE: To provide a key device, which is engaged and disengaged surely and a malfunction of which can be prevented while being difficult to be damaged.

CONSTITUTION: Projections 41b-48b for positioning are formed to the inner peripheral sections of tumblers 41-48 with approximately square-shaped front faces respectively. On the other hand, keyways 102 capable of being fitted to the projections 41b-48b for positioning of the tumblers 41-48 are formed to the surface and rear of a key 100 in the longitudinal direction while a pair of tapered surfaces 104, 104 are formed at the front end section of the key 100 and formed in pointed ends. The key 100 is inserted to the tumblers 41-48, and the inner peripheral sections of the tumblers 41-48 are pushed and slid by the tapered surfaces 104, 104. The projections 41b-48b for positioning of the tumblers 41-48 are fitted into the keyways 102 of the key 100.