A tire pressure information monitoring system includes a pressure sensor (24) for detecting air pressure within a tire, and a temperature sensor (25) for detecting temperature within the tire. The monitoring system further includes a CPU (21) that operates with a high frequency clock to send command to the pressure sensor (24) and the temperature sensor (25) to acquire internal information of the tire, and thereafter operates with a low frequency clock until stabilizing time of the pressure sensor (24) and the temperature sensor (25) elapses. The CPU then operates with a high frequency clock to transmit the internal information of the tire, which has been acquired by the pressure sensor (24) and the temperature sensor (25), to an external receiver at a constant time interval, and thereafter operates with a low frequency clock until the constant time interval elapses.