A reference voltage generating unit generates a plurality of analog reference voltages, and an A/D converting unit converts the analog reference voltages thus generated and an analog input voltage input from an external device to digital reference values. A CPU generates, based on the analog reference voltages and the digital reference values converted from the analog reference voltages, an equation for correcting the analog input voltage to be converted to a digital value falling in a range of the digital reference values. With the equation generated, the CPU calculates the analog input voltage for the digital value obtained by conversion.