An apparatus includes an acceleration calculation means for calculating, based on rotation speed information obtained from a rotation speed detection means, accelerations of the respective tires; an acceleration variation calculation means for calculating acceleration variations of the tires; a comparison means for comparing the acceleration variations; a presumption means for presuming a pressure decrease of a tire; an examination means for identifying, based on the tire rotation speed information, a tire having a rotation speed that is higher than the rotation speeds of the other tires or that is lower than a predetermined criterion; and a determination means for determining, when a tire identified by the presumption means is identical with a tire identified by the examination means, a pressure decrease of the tire. The the predetermined criterion defines a degree at which, when four tires have a normal internal pressure and one of the four tires is fitted with a snow chain, the tire has a lower rotation speed than the rotation speeds of the other tires. The examination means is configured to identify a tire having the slowest rotation speed. The determination means is configured to determine that a tire has a decreased air pressure when the tire identified by the presumption means is identical with the slowest tire identified by the examination means and when the slowest tire is positioned at a driven wheel.