To provide a tire state monitoring device capable of easily registering the ID codes of transmitters in a receiver.

When the ID codes of the transmitters 3 are registered in the receiver 4, the operating switch 7 of a commander 5 is pressed with the commander 5 positioned near the specified transmitter 3. Then instruction signals are sent from the commander 5 to the transmitters 3. The transmitters 3 transmit response signals eight times continuously according to the instruction signals. When receiving signals from the transmitters 3, the receiver 4 measures the received times of signals. When the received times of signals is eight, the receiver 4 registers the ID codes included in the received signals. Namely, the receiver 4 determines based on the received times of signals whether the ID codes during signal receiving are registered or not.