To provide a device for giving an alarm on the drop of the air pressure of a tire capable of preventing any erroneous alarm or no alarm by correcting the relative speed ratio of a traveling drive wheel from the relationship between the relative speed ratio of right and left dissimilar tires of the drive wheels and the drive force to judge the drop of the internal pressure of the tire.

A wheel speed sensor 1 detects the rotational speeds V1-V4 of tires W1-W4, a control unit 2 achieves the operation by the prescribed control program to display the tire whose air pressure is dropped on a display 3. A case is considered where one tire is renewed with a new tire when the drive wheel tires W3, W4 are worn by approximately 50%. As the initializing work, the data on the right and left wheel speed ratio V3/V4 of the drive wheels and the forward/rear wheel speed ratio (V3+V4)/(V1+V2), and the right and left wheel speed ratio V3/V4 of the drive wheels is obtained through the regression analysis. The right and left wheel speed ratio V1/V2 of the driven wheels is separately obtained. In the pressure drop judging process, the data of the wheel speed V1-V4 per second is corrected by the right and left wheel speed ratio of the drive wheel and the driven wheel, and then, the value obtained by the prescribed arithmetic formula is compared with the threshold for judgment.