In the signal quality measurement device of the present invention, a spectrum measurement circuit (101) includes: an N-(where N is an integer equal or greater than 2) phase clock generation circuit (304) for supplying phase-modulated signals in which the phase of a clock signal is shifted by a phase modulation amount each time the settings of the phase modulation amount are switched; a mixer circuit (303) for taking the product of a measured signal that is supplied from a transmitter and the phase-modulated signals that are supplied from the N-phase clock generation circuit (304); an average value output circuit (305) for supplying an average voltage value of the output signal of the mixer circuit (303); a memory (307) for storing the average voltage value supplied from the average value output circuit (305) for each phase modulation amount of the N-phase clock generation circuit (304); and arithmetic unit (308) for using the average voltage value for each phase modulation amount of the N-phase clock generation circuit (304) that is stored in memory (307) to calculate the signal strength of the measured signal.