PURPOSE: To smoothly take proper measures at the time of fault generation by reducing the burden of confirming the zero position and the span deviation in the analog signal input device and the soundness of the device.

CONSTITUTION: In the input device which a multiplexer 4 successively switches and inputs plural analog signal inputs 1 and are converted into digital values by an A/D converter 5, beforehand the reference voltage signal from a reference voltage generating device 3 is converted into the digital signal to be stored in E2PROM 10 as the value for the zero point adjustment and for the span adjustment. In the idle intervals between the conversion and transmission for the normal signal input, a signal switching device 2 is switched and connected to the side of the reference voltage generating device 3, obtaining the value adjusted by means of the input adjustment value for the zero point adjustment and for the span adjustment held in the E2PROM 10. The level difference between the value and the initial value which is previously written in the E2PROM 10. When the difference is larger than the allowable range, the information is transmitted to a main computer 14, generating a warning of abnormality or the like.