Data representing a ground contact time of a tire is transmitted from a tire side device, and received by a vehicle side device to calculate the ground contact time of the tire, to thereby calculate a ground contact length of the tire at that time. In a situation where a tire air pressure may change such as a tire exchange, the ground contact length at that time is set as a reference ground contact length, and a determination reference value is set on the basis of the reference ground contact length. As a result, the relative determination reference value corresponding to the tire type can be set, and a variation of the reduction rate of the tire air pressure when it is determined that the tire air pressure is reduced can be reduced. As a result, the reduction in the tire air pressure can be detected with high precision regardless of the tire type.