A device, and an analogous method and program, for detecting whether different tires are mounted on the left and right driving wheels of a vehicle and accurately detecting decreased tire pressures. The device detects the rotational speeds of wheels mounted on the vehicle, detects torque applied to driving wheels, and calculates the inclinations of straight lines representing (1) the relation between the rotational speed ratio of the front and rear left side wheels and torque applied to driving wheels and (2) the relation between the rotational speed ratio of the front and rear left side wheels and torque applied to driving wheels. The device compares the inclinations of the lines to detect whether different tires are mounted on the driving wheels. Based on the comparison of the inclinations, the device changes the threshold for judging tire deflation to different values for different tires and tires that are not different.