An object is to provide a method capable of reducing calculation number of times for obtaining a primary regression line of a slip ratio and acceleration, and detecting a decrease in internal pressure of tires with high accuracy even in the case where is no evenness in a road surface.The present invention is a method for alarming decrease in tire air-pressure including the steps of: detecting rotational information obtained from tires installed in a vehicle; respectively calculating a wheel velocity, a vehicle acceleration and a slip ratio of the front wheel and a slip ratio of the rear wheel from the rotational information of the respective tires; accumulating the vehicle acceleration and the slip ratio of the front wheel and the slip ratio of the rear wheel; obtaining a primary regression coefficient of the vehicle acceleration and the slip ratio of the front wheel and the slip ratio of the rear wheel; and judging whether or not the tires are decreased in the air pressure upon comparing the primary regression coefficient and a reference value of the primary regression coefficient preliminarily calculated in the above step when air pressure in the tires is the reference internal pressure.