To observe a wave with a longer wavelength to miniaturize a device, and to reduce costs.

In this method, a transmission radio wave (3) is applied to waves (1 and 2), the reflected wave is received (1, 2, and 5), the wave is observed (6). In this case, a carrier wave (3) is modulated by a modulation signal (4), and the cycle of the modulation signal (4) is set so that a wavelength being determined according to the cycle of the modulation signal (4) and the propagation speed of the radio wave and being used as a modulation wave doubles or nearly doubles as compared with the wavelength of the wave to be observed.