A system detects deflated tires based on a resonance frequency of tires with a vehicle and a relative comparison value of rotation speeds of the tires. The system includes: a rotation information detection means for detecting rotation information of the tires; a rotation speed calculation means for calculating rotation speeds of the tires; a resonance frequency assumption means for assuming a resonance frequency of the tires; and a determination means for detection of deflated tires using a decreased pressure determination index calculated by a relative comparison of the rotation speeds of the tires and the resonance frequency. The determination means is so configured as to reduce the second threshold to be compared with the decreased pressure determination index to a value equal to or higher than 1% and equal to or lower than 90% and to detect deflated tires when the decreased pressure determination index exceeds the reduced threshold.