PURPOSE: To obtain correct digital conversion data without making offset adjustment, etc., by storing offset data for each output of a multiplexer and gain correcting coefficient data in a memory.

CONSTITUTION: A switch 300 is turned off and an output of an amplifier 20 is applied to an AD converter 30, and an output of the AD converter 30 is applied to a controller 600. Under this condition, output of the AD converter 30 obtained by making input signals E1, E2... as zero is made offset data of each channel. Next, the switch 300 is closed and offset voltage is given to the amplifier 20. Full-scale voltage is set to input signals E1, E2... and gain correcting coefficient data are calculated from digital value of output of the AD converter. Both above-mentioned data are stored for each channel in a memory 100 and read out at the time of channel selection to obtain correct AD conversion value.