To confirm the state of the locking or unlocking of a cylinder lock by using a key.

When a key 2 inserted into a keyhole 110 for the cylinder lock 1 is rotated to a locking area Z1, a voltage indicating the locked state detected by a locking detecting section 111 or the state signal of pulses is transmitted in an electromagnetic induction manner from a transmitting section 113. The locking of the cylinder lock is decided by a state decision section 201 in the key receiving the state signal in the electromagnetic induction manner by a receiving section 200, and the time decided as the result of a decision is stored in a storage section 203 while being displayed to a display section 204. Then, when the key inserted into the keyhole is rotated to an unlocking area Z2, the voltage indicating the unlocked state different from the locked state detected by an unlocking detecting section 112 or the state signal of pulses is transmitted in the electromagnetic induction manner from the transmitting section. The unlocking of the cylinder lock is decided by the state decision section in the key receiving the state signal in the electromagnetic induction manner by the receiving section, and the time decided as the result of the decision is stored in the storage section while being displayed to the display section.