Compatibility with a current transformer having several kinds of rated current ratios is achieved by finding out a formula of relationship between errors arising in a transformer input circuit and detectable ranges set as setting values of the input circuits which can be optimally set. In connection with that, by dispensing with a gain control section, the number of components, the mounting area and the cost can be reduced. Arithmetic expressions representing the relationship among errors arising in the transformer input circuit, the instrumentation ranges of the input circuits, and detectable errors have been derived. Further by optimally setting the detectable ranges of the input circuits, which are the parameters of the arithmetic expressions, the configuration compatible with a current transformer having several kinds of rated current ratios is achieved.