To provide a phase measuring method and a phase measuring instrument, by which a phase of a peak signal such as an Lpp signal is correctly measured.

A signal decimated by a 1/128 decimation part 15 is orthogonally detected by an orthogonal detection part 20, and the orthogonally detected signal is converted into a complex base band signal of approximately zero Hz by Δθ/Δt frequency calculating parts 22, 25 and frequency shift parts 23, 26 based on a frequency of the signal. On the other hand, a position of the Lpp signal to data of a memory 14 is determined by an FET part 16, a peak detecting part 17, a threshold level determining part 18 and an Lpp signal position determining part 19, and the phase of the Lpp signal is detected based on the position of the Lpp signal and the complex base band signal.