In the present invention, a main generation unit (11) generates, by using a phase signal (51), a first sawtooth wave signal (501) synchronized with an ideal function. An adjustment unit (121) generates an adjustment signal (604) by multiplying the phase signal (51) by a constant and by further adding a constant thereto. A sub-generation unit (122) generates, by using the adjustment signal (604), a second triangular wave signal (602) which has the same phase and frequency as an error correction signal (603) for offsetting an error function representing an error with respect to an approximate triangular wave signal generated from the first sawtooth wave signal (501). A second correction unit (123) generates the error correction signal (603) by adjusting a value range of the waveform of the second triangular wave signal (602). An addition unit (113) generates a corrected sawtooth wave signal (501a) by adding the first sawtooth wave signal (501) and the error correction signal (603). An approximate signal generation unit (114) generates an approximate signal (52) by using the corrected sawtooth wave signal (501a).