A calibration method for improving distortion of a waveform of a point-spread-function without constantly executing feedback control to a wavelength-swept light source is provided. An interference signal is generated by varying voltage to be applied to a light source within one period, the interference signal is sampled at equal time intervals on a time axis, the point-spread-function is obtained through Fourier transform by multiplying by a first window function, and a complex analysis signal including frequency information of light is obtained through inverse Fourier transform by multiplying the point-spread-function by a second window function. After a variation in a frequency of the light relative to a time within one period is obtained at equal time intervals by unwrapping phase information of the complex analysis signal, a correspondence relationship between the variation in the frequency of the light within one period and a variation in voltage within one period is obtained.