The rotational speed difference between rotational speeds of front and rear road wheels of a motor vehicle is weighted by a first ratio, and the difference between the rotational speed difference between rotational speeds of front left and right road wheels and an average signal of reference steering angle or between the rotational speed difference between rotational speeds of rear left and right road wheels and the average signal of reference steering angle is weighted by a second ratio. A sum signal representative of the sum of the rotational speed difference weighted by the first ratio and the difference weighted by the second ratio is used to determine a pneumatic tire pressure reduction highly accurately even when running conditions of the motor vehicle change such as when the motor vehicle makes a turn.