PURPOSE: To obtain the synthesizer with less frequency fluctuation of an oscillator at the time of switch changeover, a low spurious characteristic and small C/N deterioration by using an amplifier type switch in its linear operating region and arranging an attenuator to a pre-stage and a post-stage of the amplifier.

CONSTITUTION: An attenuator 2, an amplifier 3, an attenuator 4, an amplifier type switch 5 and an SPST switch 6 are connected in cascade to an output of an oscillator 1, and an output of the SPST switch 6 is connected to a 1st input terminal 8 of an SPDT switch 7. An attenuator 10, an amplifier 11, an attenuator 12, an amplifier type switch 13 and an SPST switch 14 are connected in cascade to an output of an oscillator 9, and an output of the SPST switch 14 is connected to a 2nd input terminal 15 of the SPDT switch 7. Then the amplifier type switches 5,13 are operated in the linear operating region, and an attenuation of the attenuator is decided between the amplifier type switch 5(13) and the oscillator 1(9) so as to obtain a desired output and the attenuation is distributed by the attenuators 2,10 and 4,12.