PURPOSE: To prevent erroneous function due to an operational region in the way of variation when the operation results varies with time toward a fault point due to the influence of a filter, for example, upon occurrence of an external fault.

CONSTITUTION: A suppression current input processing section 26 and a differential current input processing section 27 are provided with delay processing sections 60, 61 where a decision is made whether switching is made between the delayed input and a normal input. The characteristics varies such that the locus of fault does not pass through an operational region thus preventing erroneous function. In this regard, the input is switched by discriminating between the occurrence of fault and the removal of fault based on the varying direction of the magnitude of current.