A method for detecting decrease in tire air-pressure in which decrease in internal pressure of a tire is detected on the basis of wheel speed information obtained from wheels attached to a vehicle. The method includes the steps of: detecting wheel speed information of the respective tires; storing the wheel speed information of the respective tires; obtaining a vehicle body velocity of the vehicle; obtaining vehicle information of the vehicle during running such as driving force, wheel torque or slip rate; calculating a judged value of decompression; and detecting decrease in air-pressure of a tire upon calculating a multiple correlation coefficient for the judged value between the vehicle body velocity and the vehicle information and comparing the multiple correlation coefficient with a specified threshold. It is possible to accurately detect decompression of a tire.