To enable precise calculation of a propagation time of a direct wave which arrives fastest from an ultrasonic generation source without being affected by a reflected wave of an ultrasonic signal.A plurality of ultrasonic signals generated based on a plurality of M sequences different from each other are propagated from a transmission side to a reception side, whereby the reception side generates model waveforms of the plurality of M sequences, executes correlation processing between each model waveform and a waveform of each ultrasonic signal and detects a secondary peak of a correlation value which appears when both waveforms partly coincide with each other to determine an M sequence which generates a smallest secondary peak among secondary peaks detected by all the correlation processing as an optimum M sequence. The transmission side sends out an electromagnetic signal and an ultrasonic signal modulated by an optimum M sequence generated based on an initial condition of the optimum M sequence to the reception side. The reception side calculates a correlation value between the ultrasonic signal and a model waveform of the ultrasonic modulated by the optimum M sequence, detects a first primary peak of the calculated correlation value and calculates an ultrasonic propagation time from a time point of the reception of the electromagnetic signal and a time point of the detection of the primary peak.