PURPOSE: To make an output signal level of a sweep signal generating means variable, by providing a sweep signal generating means for generating a sweep signal in an asynchronous state of a phase control loop, and operating it by turning-on and -off operation of a manual switch.

CONSTITUTION: A reference oscillator 51, a phase comparator 52, a low-pass filter 53 and an adder 54 constitute a phase control loop for controlling an oscillation frequency of a local oscillator 2 so that frequency of an output signal of a frequency converter 1 which has been amplified to a prescribed amplitude by an intermediate frequency amplifier 3 coincides with an oscillation frequency of the reference oscillator 51. A sweep signal generator 55 is coupled with the adder 54 through a manual switch 56, and when the phase control loop is in an asynchronous state, it starts operation and operates so as to maintain a synchronizing state. The local oscillator 2 is controlled by an AC component of an output signal of the sweep signal generator 55. When the switch 56 is opened, the oscillator 2 is controlled by only the phase control loop.