A system for monitoring a tire pressure includes a sensor unit and a monitoring unit. The sensor unit is mounted on a tire of a vehicle to detect an air pressure in the tire. The monitoring unit is mounted on the vehicle to monitor a signal indicative of the air pressure detected by the sensor unit so as to watch a decrease in the air pressure. The sensor unit includes an accelerometer so as to detect an acceleration acting on the tire. And the monitoring unit classifies the tire as one of a rolling tire and a spare tire based on a signal indicative of the acceleration transmitted by the sensor unit.