An object of the present invention is to propose a design method of a minimum value N of the number of multiplexed frequencies that is necessary for measuring an object vibrational frequency according to DAS-P while taking into consideration a trade-off between a measurement distance and an upper limit of a measurable vibrational frequency. When a phase change of an arbitrary section of a measured optical fiber is represented by A × sin (2nfvt), N = 4Zfv/v is satisfied when A is smaller than n/2 but N = 2Zfv/(ν·Arcsin (π/2A)) is satisfied when A is equal to or larger than n/2, where fv represents vibrational frequency, t represents time, Z represents a length of the measured optical fiber (a measurement distance), and v represents the speed of light inside the measured optical fiber.