PURPOSE: To eliminate system error by providing means for regulating the variation rate of first ramp voltage connected with a first voltage generating means and a ramp count means.

CONSTITUTION: Sequential repetitive calculation is performed over several calibration cycles until a correction coefficient (dk) in an up/down counter 138 goes zero. Consequently, system calibration is ensured. The calibration circuit adds less than 200 gates to the circuit shown on the drawing and generates current correction coefficients in leading mode. The calibration is independent from the signal propagation lag (Tα1, Tα2), the charging/discharging characteristics (Tw1, Tw2) and the nonlinearity of a ramp generator. Nonlinearity at the linear part of ramp has effect not on the calibration of range but only on the linearity of timing resolution. Consequently, system error is eliminated.