Once a sensor amplifier side connector of a cable unit for interconnecting a detector and a sensor amplifier is connected to the sensor amplifier, a gain resistor on the cable unit side and an instrumentation amplifier of the sensor amplifier are interconnected and the gain of the instrumentation amplifier is determined. As a result, the gain of the instrumentation amplifier is determined such that the magnitude of a signal that the sensor amplifier receives from the detector becomes a magnitude optimal for an analog circuit in the sensor amplifier.