PURPOSE: To reduce a conversion error in the case of calibrating the sensitivity of the generator by applying a calibration signal to the generator having an A/D converter so as to adjust the conversion output.

CONSTITUTION: A DC voltage DC1 with a reference level is generated from a DC standard power supply 11. A triangle wave signal S1 is generated from a superimposing wave signal generating circuit 15 as a superimposing wave signal. Thus, a signal S2 resulting from superimposing the triangle wave signal S1 onto the DC voltage DC1 with a reference level is obtained at an output terminal 10a of the calibration signal generator 10. The calibration signal S. inputted to an input terminal 20a of a digital memory device 20 is inputted to an input amplifier 21 and its offset and gain are adjusted by adjusting the resistance of variable resistors 22, 23.