The apparatus for detecting a decreased air pressure of a tire includes an acceleration calculation means for calculating, based on rotation velocity information of tires of respective wheels of a vehicle;
an acceleration variation calculation means for calculating acceleration variations of the respective tires; a comparison means for comparing the acceleration variations of the respective tires; an estimation means for estimating, when a comparison result of the comparison means shows that an acceleration variation of a certain tire is greater than acceleration variations of the other tires, a decrease in air pressure of the certain tire; a test means for determining a tire having a rotation velocity that is faster than the rotation velocities of the other tires or that is further slower than a predetermined reference; and a judgment means for judging, when a certain tire identified by the estimation means is identical with the tire identified by the test means, a decrease in air pressure of the certain tire.