A device for finding correction factors for correcting outputs of rotational velocity detectors used for detecting the rotational velocities of four tires mounted on a vehicle. The device judges whether or not the vehicle is traveling linearly. If the vehicle is traveling linearly, an initial correction factor K.sub.1 for eliminating the effect of a difference in effective rolling radius between the right and left front tires, and an initial correction factor K.sub.2 for eliminating the effect of a difference in effective rolling radius between the right and left rear tires, are calculated on the basis of the outputs of the rotational velocity detectors. When it is judged that the vehicle is coasting linearly, an initial correction factor K.sub.3 for eliminating the effect of a difference in effective rolling radius between the front tire and the rear tire is calculated.