According to the present invention, there is provided a tire monitoring system in which electric power is fed from a car body to a tire sensor unit in a non-contact manner. The tire sensor unit 10 (10a-10d) is mounted on a corresponding tire 2 (2a-2d) of a car, and electric power is radio-transmitted from a corresponding non-contact type power supply portion 40 (40a-40d) provided in the car body to the tire sensor unit 10 by electromagnetic induction or by microwaves. The tire sensor unit 10 is provided with a non-contact type power receiving portion which generates direct-current power by energy transmitted from the non-contact type power supply portion 40 so as to supply electric power necessary for activating the tire sensor unit 10. The tire sensor unit 10 senses air pressure or the like, and radio-transmits the information to the receiving device 20.