To reduce considerably an undesired spurious component due to periodic fluctuation in a duty ratio without use of a ROM.

In the frequency synthesizer changing a frequency of a signal obtained at an output by changing input data ΔPHA of an accumulator 1 accumulating input data for each block, output data of the accumulator 1 are integrated by an integration device 4 and the integration output is compared with a comparison voltage by a comparator 5. An integration coefficient is changed by output data of the accumulator 1 and the ΔPHA in the integration device 4 and the integration is started by a carry out signal from the accumulator 1.