An object of the present invention is to provide a device for detecting as to whether or not the heterogeneous tires are set on the right and left wheels, respectively, and accurately detecting the loss of the pneumatic pressure in the tire, even when heterogeneous tires are set on the right and the left wheels, respectively. This is a method for detecting as to whether or not the kinds of the tires set on a vehicle are different based on the rotational velocity of the wheels set on the vehicle, so that, when the pneumatic pressure of the tire set on said vehicle is normal, the velocity ratios of the right and left front wheels or the velocity ratios of the right and left rear wheels are obtained by each velocity region of the vehicle as described above, and comparison is made between the wheel velocity ratios of the right and left front wheels by each velocity region or between the wheel velocity ratios of the right and left rear wheels by each velocity region, thereby detecting that the heterogeneous tires are set between the right and left sides. Furthermore, the method for detecting loss of the pneumatic pressures of tires according to the present invention is designed to modify the threshold value for judging loss of the pneumatic pressures of the tires based on the results detected by the method for detecting the heterogeneous tires as described above.