An apparatus for detecting a decrease in air pressure of a tire attached to a vehicle. The apparatus includes a rotation speed detection means for periodically detecting rotation speeds of tires of respective wheels of the vehicle; a rotation wheel speed ratio calculation means for calculating, based on rotation speed information obtained by the rotation speed detection means, a rotation wheel speed ratio between front wheels and rear wheels of the vehicle with regard to a plurality of predetermined speed regions; a wheel torque calculation means for calculating a wheel torque of the vehicle; an initialization means for obtaining, with regard to the respective speed regions, a relation at a normal internal pressure between the wheel torque and the rotation wheel speed ratio between the front wheels and the rear wheels; a storage means for storing the relation obtained by the initialization means; a comparison means for comparing, with regard to the respective speed regions, the rotation wheel speed ratio obtained by the rotation wheel speed ratio calculation means with the rotation wheel speed ratio at a normal internal pressure obtained from the wheel torque obtained by the wheel torque calculation means and the relation stored by the storage means; and a determination means for correcting the comparison result by the comparison means by a front-to-rear direction acceleration and for determining, based on the resultant corrected value, whether there is a tire having a decreased air pressure or not.