A differential amplifier (11) differentially amplifies an inputted burst signal. A first maximum value holding circuit (12) detects and holds the maximum value of an inverting output of the differential amplifier (11). A second maximum value holding circuit (13) detects and holds the maximum value of a non-inverting output of the differential amplifier (11). A differential amplifier (14) differentially amplifies outputs of the first and second maximum value holding circuits (12) and (13). An output of the differential amplifier (14) is fed to a differential amplifier (15) as a reference input. The differential amplifier (15) differentially amplifies the non-inverting output of the differential amplifier (11) with the reference input from the differential amplifier (14) used as a basis. The first and second maximum value holding circuits (12) and (13) have the same circuit design, and the sizes and the directions of variations respectively occurring inside thereof are the same. The differential amplifier (14) is so constructed that the amplification factor thereof is set to 0.5, and a variation occurring inside thereof is the same as a variation occurring inside of the differential amplifier (11). Respective output variations occurring in the maximum value holding circuits (12) and (13) due to a temperature variation and a power supply voltage variation are canceled by providing differential amplification in the differential amplifier (14). At this time, an output variation occurring in the differential amplifier (11) is also canceled. Therefore, an output variation occurring in the differential amplifier (14) is made equal in value to the output variation occurring in the differential amplifier (11), so that the same variation as a signal input is superimposed on the reference input fed to the differential amplifier (15). Consequently, a reference input accurately following the DC level of the signal input can be fed to the differential amplifier (15), whereby stable amplification can be always provided.