An apparatus for detecting a decrease in tire pressure based on a resonance frequency of a tire installed in a vehicle includes rotation information detection means for detecting rotation information of the tire, resonance frequency estimate means for estimating the resonance frequency of the tire from the rotation information of the tire, and judgment means for judging the decrease in the tire pressure by comparison between the resonance frequency estimated by the resonance frequency estimate means and a reference frequency preliminarily determined by learning at the time of initialization. An effective speed region when the reference frequency is determined at the time of the initialization is a first speed or less. The apparatus further includes effective speed region change means for extending an upper limit of the effective speed region to a second speed which is larger than the first speed at the time of shifting.