Not affected by the reflected wave of the ultrasound signal, it makes it possible to accurately calculate the direct wave propagation time to reach the earliest from the ultrasonic source.
Is transmitted from a sender to a receiver a plurality of ultrasound signals generated based on a plurality of different M-sequence, the receiving side generates a model waveform of a plurality of M sequence, the model waveform and the waveform of the ultrasonic signal and performing a correlation processing with the to detect the secondary peak of the correlation value appearing when the both waveforms is partially coincide, generates a minimum of secondary peak of the secondary peaks detected in all correlation to determine the M-series as the best M series. Sender sends an electromagnetic wave signal, an ultrasonic signal modulated by the generated optimum M sequence based on the initial condition of the optimum M sequence to the reception side. The receiving side calculates a correlation value between a model waveform of the ultrasonic modulated by the ultrasonic signal and the optimum M sequence, the first main peak of the calculated correlation value is detected and received electromagnetic signals calculating the ultrasonic wave propagation time from the detection time point of time and the main peak.