To provide a tire pneumatic pressure monitoring device capable of continuously monitoring the tire pneumatic pressure while suppressing consumption of a battery of a tire pneumatic pressure sensor unit provided in a wheel.

When a traveling state determination operation unit of a first tire pneumatic pressure monitoring device 2 determines that a vehicle C is in a stably traveling mode, the stop signal is output to a sensor side transmission-reception unit in each tire pneumatic pressure sensor unit 22 from each vehicle body side transmission-reception unit 23, each tire pneumatic pressure sensor unit 23 is turned OFF, and the tire pneumatic pressure of each wheel W is monitored by a first tire pneumatic pressure monitoring device 2. When it is determined that the vehicle W is not in a stably traveling state, the start signal is output to a sensor side transmission-reception unit in each tire pneumatic pressure sensor unit 22 from each vehicle body side transmission-reception unit 23, each tire pneumatic pressure sensor unit 23 is turned ON, and the tire pneumatic pressure is monitored by a second tire pneumatic pressure monitoring device 3.