To reduce influence from a load and to enlarge a frequency band by providing a power divider for branching output into two, an amplifier for the amplification of one of output signals and the amplifier for feeding back and outputting the other output signal.

To the conventional constitution of a variable attenuator 20, a detector 50 and an integrator 60, the power divider 30 and the amplifiers 41 and 42 are added. The power divider 30 divides signal power by providing the impedance matching of input and output. For instance, in the case of using the three resistors of 16.6 Ω, the power divider of impedance 50 Ω and an attenuation amount 6dB is attained. The amplifiers 41 and 42 are provided with isolation characteristics for amplifying signals in a forward direction and attenuating the signals in an opposite direction. Since the isolation of feedback signals and reflection signals is enlarged, the influence from the load is reduced. Also, in the case of using the power divider for the branching of the signals, the frequency band is widened as well.