To easily perform adjustment in a short period of time to turn on a carrier detection comparator at a level to detect a carrier signal.

The carrier detection comparator 1 is provided with hysteresis characteristics, where a comparison voltage in changing from 'on' to 'off' is different from that in changing from off to on. A reference voltage is inputted to the non-inverted input end of the carrier detection comparator 1. The comparison voltage, obtained by wave-detecting an adjustment signal which is outputted from a signal generator 3, is inputted to the inverted input end of the comparator 1. The level of the adjustment signal is changed periodically between the level of the carrier to be detected and the level, which is lower than the level of the carrier to be detected and which will not become at least more than the range of the hysteresis. A voltage at the output end of the comparator 1 is monitored by a needle voltage indicator 5. The reference voltage is fixed when the deflection of a needle is changed from one stable position in the needle voltage indicator 5, after changing the reference voltage.