A reference signal from a reference signal source is supplied to a frequency transformer and a side band signal generator. The frequency transformer is formed mainly by up-converters and produces a frequency m times as high as the frequency f.sub.s of the reference signal. The side band signal generator outputs signals of base and harmonic frequencies f.sub.s, 2f.sub.s, 3f.sub.s, . . . and nf.sub.s (where n is less than m) and produces less phase noise than does the frequency transformer. The outputs of the frequency transformer and the side band signal generator are frequency mixed by a frequency mixer, and one frequency component in the frequency-mixed output is selected by a variable filter.