PURPOSE: To correct the offset of each channel by subtracting the A/D conversion value of an analog input to a channel connected to ground as an offset from an A/D conversion value of another channel.

CONSTITUTION: A digital output 3a to be an A/D conversion value obtained by A/D converting an analog input 1n to a channel CHn by means of an A/D converter 3 is inputted to a computer 13 as an offset. Then a switching part 10A in a switching means 10 is switched to an input part 10a to obtain an analog input 1a to another channel CH1, the analog input 1a is converted by the A/D converter and a digital output 3a to be an A/D conversion value is inputted to the computer 13 as a digital value input 3aA. Thereby the computer 13 can correct an offset for the A/D conversion value of the analog input 1a by subtracting the A/D conversion value of the analog input 1n from that of the analog input 1a.