An apparatus for detecting a tire having a decreased pressure based on resonance frequency of tires attached to respective wheels of a vehicle. The apparatus includes a storage means for storing information regarding a distribution of resonance frequencies respectively corresponding to a plurality of air pressure statuses, an initialization means for estimating a frequency characteristic of the rotation speed information or the rotation acceleration information, a frequency estimation means for estimating a frequency characteristic of the rotation speed information or the rotation acceleration information of a running vehicle, and a Bayesian estimation means for subjecting a tire pressure status at a certain time to a Bayesian estimation based on the resonance frequency during the initialization, a resonance frequency at the certain time obtained from the frequency estimation means, and distribution-related information stored in the storage means.