To provide a measuring method of the ratio of a signal to a noise and a measuring device of the ratio of the signal to the noise capable of measuring the ratio of the signal to the noise highly accurately at high speed.

An FFT part 16 executes fast Fourier transform of a wobble signal to generate spectrum, and a peak detection part 17 determines a peak level from the spectrum. The first level histogram formation part 18 and the second level histogram formation part 19 form a level histogram from the peak level, and a noise floor detection part 21 detects a noise floor from the level histogram. A calculation part of the ratio of the signal to the noise 22 determines the ratio of the signal to the noise from the peak level used also for detection of the noise floor and the detected noise floor.