A tire pressure monitoring system is capable of ensuring accuracy of the rate of transmission of data via wireless communications without an increase in the number of oscillators and an increase in the cost. The tire pressure monitoring system includes a tire pressure measuring module. The tire pressure measuring module has a microcomputer, an activation control circuit, a pressure sensor, a temperature sensor, a frequency divider, a transmitting circuit, and a battery. The microcomputer has a clock pulse generator, analog-to-digital converter circuits and a controller. The transmitting circuit has an oscillator circuit. The frequency divider divides the frequency of a carrier wave CW output from the oscillator circuit to generate a clock signal, and outputs the clock signal to the controller included in the microcomputer. The clock signal is used for the timing for outputting data to a data signal line DL. The clock signal for high precision data transmission can be generated without the need to provide an expensive oscillator in the microcomputer since the oscillator circuit is used in the transmitting circuit, and the clock signal used for data transmission by the microcomputer is generated by the inexpensive frequency divider.