A waveform conversion unit (42) of a numerically controlled oscillator has a cosine table (101) and a sine table (102) in which parameters for cosine wave and sine wave signal generation are stored, a correction mechanism (50) for generating correction values according to the phase of an inputted phase signal, an adder (111) for generating a cosine wave signal from a parameter in the cosine table (101) and a correction value, and an adder (112) for generating a sine wave signal from a parameter in the sine table (102) and a correction value. The correction mechanism (50) uses waveform data which is a kind of parabolic data and whose phase interval is more minute than the phase interval of the parameters in each table (101), (102) to generate correction values for correcting cosine wave and sine wave signals to be found by linear interpolation.