An apparatus includes: an offset adjustment unit supplying an offset correction signal corresponding to a frequency switching to an adder unit receiving output from a mixer; a timing adjustment unit adjusting the timing of a frequency switching signal supplied to a local oscillator and the timing of an offset correction amount switching signal supplied to the offset adjustment unit for changing an offset amount in correspondence with the frequency switching in the local oscillator; a noise amount measurement and calculation unit receiving a signal obtained by amplifying and filtering the signal from the adder unit, to measure a noise amount of the signal and generates a timing determination signal based on the noise amount; and a control unit controlling frequency switching signal timing and the offset correction amount switching signal supplied to the timing adjustment unit, based on the timing determination signal from the noise amount measurement and calculation unit.