To provide a tire pneumatic pressure monitoring system and a tire pneumatic pressure monitoring method, which improve the communication establishment between a tire air pressure signal and a receiver for the signal when the tire air pressure signal is transmitted from a sensor unit monitoring a tire air pressure.

Both of a UHF (Ultra High Frequency) transmitter 15 and a UHF receiver 16 are provided to an vehicular electronic key 2, so that radio waves in a UHF band can be communicated interactively. Then, the electronic key 2 is used as a relay (repeater) of a tire pneumatic pressure monitoring system 22. In particular, when the electronic key 2 receives tire pneumatic pressure signals Stp from a sensor unit 23 of the tire 21, the electronic key 2 transmits all signals to a vehicle tuner 10 after receiving all of the signals Stp. Thereby, the establishment of reception of the tire pneumatic pressure signals Stp is improved.