PURPOSE: To make sure that an analog value is converted correctly to a digital value, by canceling an effect of the analog value in a power system, and comparing a digital conversion value with a previously set value.

CONSTITUTION: A checkup superimposing circuit 13 for superimposing a checkup input into an analog electric value received from a power system, an analog value canceling circuit 171 for canceling a previous analog value when the checkup input is superimposed, an instantaneous current value comparing circuit 174 for comparing a digital value converted from the analog value in input checking with a forecasting digital output value calculated from a known superimposed output, and an appropriateness discrimination circuit 175 for receiving this output from the comparing circuit 174 and discriminating the appropriateness are provided. The checkup input is judged to be good in input checking after the checkup input in each sampling time unit is certified as correct for a given time.