A microwave frequency converter for a radar receiver is provided in which a frequency of a local oscillator of the microwave frequency converter can be prevented from interfering with an oscillation frequency of a magnetron included in the radar receiver and which can receive a signal from a considerably short distance. A voltage of an RF amplifier connected to the microwave frequency converter is synchronized with an oscillation output of the magnetron. The gate and drain voltages of a FET used in the RF amplifier are simultaneously switched OFF only during times before and after the magnetron performs oscillation, thereby switching an amplification function of the FET to an attenuation function thereof to increase a switching loss of the microwave frequency converter. Thereby, an excessively large power RF signal directly input from the magnetron and an excessively large power RF signal reflected from a short distance are attenuated to prevent the microwave frequency converter from being saturated, and the frequency of the local oscillator from interfering with the oscillation frequency of the magnetron.