PURPOSE: To provide an input correction method for an A/D converter with which correction is facilitated with high performance by preparing a correction relation based on the input data value of the A/D converter and the reference value of input data.

CONSTITUTION: At the time of correction, a microcomputer supplies an instruction through an interface 19 to a reference value generator 9 and supplies several values to be inputted to an analog input channel 11. The imparted values pass through an operation amplifier part 12, are converted to digital data at the A/D converter 7 and are inputted to the microcomputer 15. The microcomputer 15 stores the value generated by the reference value generator 9 and the digital data at a memory device 16. When measurement at all scheduled points is completed, the microcomputer 15 forms functions to be corrected based on the data stored in the memory device 16.