To achieve both accuracy in correcting sensing error caused by nonstandard factors of a pulse signal generated by rotation of a rotor to be measured and reduction in cost of an apparatus for doing the same, each passage of rotor sensors which rotate integrally with vehicle tires are sensed to produce pulse signals. The pulse signal periods are sequentially integrated every pulse signal train consisting of multiple pulse signals. When all of the pulse signal periods of the pulse signal train are integrated, the integrated value is stored in memory. The memory always stores the integrated values of the newest one rotation of the rotor to be measured. The pulse signal periods are averaged from the sum of the integrated values. With such a construction, the average of the pulse signal periods can be obtained as a learning reference value in the correction without storing all of the pulse signal periods of one rotation, thereby realizing both correction accuracy and cost reduction.