To provide a tire pneumatic pressure monitoring system capable of preventing the other tires from receiving a request signal with a simple configuration.

The system is provided with an air pressure sensor unit 3 which is installed in each tire of a vehicle 1 measuring its air pressure and transmitting data of it by air, and an initiator which transmits a request signal to operate the air pressure sensor unit 3 with electromagnetic wave. The initiator comprises a bar antenna functioning as an antenna 15 which consists of, for example, a core in the shape of an curved arch and wire wound around the core. The antenna 15 is disposed close to a wheel housing with the both ends of the core in the direction of the air pressure sensor unit 3, and function as a transmitting antenna. The core for the antenna 15 could be channel-shaped or U-shaped other than arch-shaped.