A tire pressure monitoring system distinguishes a mounted tire from a non-mounted tire, and which automatically changes over IDs and tire-pressure thresholds without any exclusive apparatuses. A tire pressure monitoring unit stores an identification code of a tire pressure sensor unit and a tire-pressure set value as a pair. While a vehicle is traveling at a speed equal to or faster than a predetermined vehicle speed, if an identification code received within a predetermined time coincides with any one of identification codes of registered tires and the number of coinciding identification codes is equal to the number of tires, and, at least one of the coinciding identification codes is an identification code other than specified identification codes, the specified identification codes are replaced with those coinciding identification codes which are newly specified as respective identification codes of tire pressure sensor units mounted in tires mounted on the vehicle.