A method for detecting decrease in tire air-pressure in which decrease in internal pressure of a tire is detected on the basis of rotational information. The method includes the steps of: detecting rotational information of the tires; storing the rotational information; comparing rotational information of all left-sided wheels and rotational information of all right-sided wheels of the vehicle when the vehicle is performing straight-ahead running; and judging decompression on the basis of a relationship between a value obtained through comparison of the left-sided and right-sided rotational information and a specified threshold. It is possible to detect decompression of a single wheel tire while it is also possible to detect that two wheel tires on the left or right are simultaneously decreased.