To control important keys by precisely storing them and preventing them from easily taken out.

A cam plate 5 fixed to a rotor 4 of a lock unit, a lateral groove 6 penetrating in a cross direction to the main body 2 of a key constraint device, a key storing hole 7 at the main body 2 of the key constraint device located orthogonal to the lateral groove 6, a storage target key 9 located in a small- diameter portion 8 at the tip portion and inserted in the key storage hole 7, an escape notch portion 10 with a key 9 freely penetrating at one end, and a restraint notch 11 linking with the small diameter portion 8 are provided continuously. Next, a passive projection 12 linking to the cam plate at the central part is provided and a lateral long hole 13 with a rotor 4 attached at the other end is provided. Moreover, a constraint plate 14 fitted and inserted to the lateral groove 6 and spring means 15 for energizing the constraint plate 14 in an escape direction from the lateral groove 6 are provided. When locking a lock unit 3, the small diameter portion 8 of a key 9 is linked with the constraint plate 14.