A method for detecting decrease in tire air-pressure includes the steps of: detecting rotational angular velocities of respective tires; calculating an initial correction coefficient for a ratio of right and left wheels; terminating initialization of a first stage at a point of time when the initial correction coefficient has been calculated for performing a provisional judgment of decompression; calculating an initial correction coefficient for a ratio of front and rear wheels; and terminating initialization of a second stage at a point of time when the initial correction coefficient has been calculated for performing normal judgement of decompression. Detection of decompression when performing running including mostly straight-ahead running such as driving in an urban area might be made at an early stage and at high accuracy to thereby improve the performance of the system.