To provide a key taking-out prevention system capable of preventing a key from being mistakenly taken out.

The first transmitter 1 is provided in the vicinity of a storage place of the key 3, the second transmitter 2 is provided in the vicinity of a locking and unlocking section to be locked and unlocked by the key 3, and call signals are transmitted from the first and second transmitters 1 and 2 for every predetermined time. While the key 3 exists in an area enabling the call signals to receive, in the case the call signals are received, an ID signal is replied to them. The first transmitter 1 or the second transmitter 2 receiving the ID signal transmits an instrumentation starting signal to the key 3. When the key 3 receives the signal, an elapsed time from the time receiving the signal is measured, and if the elapsed time gets to the predetermined time, a warning is made. While the key 3 exists in the outside of the area enabling the call signals to receive, in the case the predetermined time is made to elapse, the warning is made.