To keep a minimum arithmetic error even when the valve opening is increased while increasing the arithmetic speed by executing tan and tan-1 by use of an arithmetic table in which the relation of a regulated tan to a regulated table is preliminarily stored.

Since the operation of a microprocessor can be performed only by judging the condition branch for successively selecting the closest value from an arithmetic table ART, the arithmetic speed is clearly increased more than the approximate calculation by Maclaurin's expansion. In order to enhance the precision, it is sufficient to simply increase the division. An angle signal θ0 is read through an angle sensor and an analog/digital converter, and the microprocessor performs the operation of tan θ0 in reference to the arithmetic table ART by use of the angle signal θ0. Thereafter, the microprocessor determines the read back value rb by use of the determined tan θ0. Since the approximate calculation by Maclaurin's expansion is not used in this case, the arithmetic speed is increased, and the arithmetic error can be minimized by increasing the division.