An amplifier for amplifying a pulse-like signal output from a secondary side of an isolating transformer, a capacitor connected to a negative feedback loop across the input and output of the amplifier, and a timing control circuit for controlling an FET into a closed state, then controlling a switch into a closed state, and after that controlling the switch into an open state at timing simultaneously with the FET or earlier than the FET are provided, and when the switch is controlled into the open state, an AD converter converts the output signal of the amplifier to a digital signal.