A measurement method, comprising: propagating an ultrasonic wave in a medium and receiving the ultrasonic wave; detecting whether or not a level of the received ultrasonic wave exceeds a predetermined trigger level L T; determining the trigger detecting period as being the peak waveform including the signal portion in which the trigger level L T is the first time exceeded; and specifying a reception timing of the ultrasonic wave based on a waveform part in a reception timing specifying period of the received ultrasonic wave different from the trigger detecting period in which the level of the received ultrasonic wave first exceeds the trigger level L T, characterized in that the method comprises the steps of: detecting a maximum peak value e from the peak values of the plurality of peak waveforms, determining the trigger level L T by multiplying the maximum peak value e with a predetermined rate, wherein the predetermined rate is determined such that a difference in peak values of the waveform part in the trigger detecting period with respect to waveform parts at front and back periods adjacent to the trigger detecting period is larger than a difference in peak values of the waveform part based on which the reception timing is specified with respect to waveform parts at front and back periods adjacent to the reception timing specifying period. propagating an ultrasonic wave in a medium and receiving the ultrasonic wave; detecting whether or not a level of the received ultrasonic wave exceeds a predetermined trigger level L T; determining the trigger detecting period as being the peak waveform including the signal portion in which the trigger level L T is the first time exceeded; and specifying a reception timing of the ultrasonic wave based on a waveform part in a reception timing specifying period of the received ultrasonic wave different from the trigger detecting period in which the level of the received ultrasonic wave first exceeds the trigger level L T, characterized in that the method comprises the steps of: detecting a maximum peak value e from the peak values of the plurality of peak waveforms, determining the trigger level L T by multiplying the maximum peak value e with a predetermined rate, wherein the predetermined rate is determined such that a difference in peak values of the waveform part in the trigger detecting period with respect to waveform parts at front and back periods adjacent to the trigger detecting period is larger than a difference in peak values of the waveform part based on which the reception timing is specified with respect to waveform parts at front and back periods adjacent to the reception timing specifying period. detecting a maximum peak value e from the peak values of the plurality of peak waveforms, determining the trigger level L T by multiplying the maximum peak value e with a predetermined rate, wherein the predetermined rate is determined such that a difference in peak values of the waveform part in the trigger detecting period with respect to waveform parts at front and back periods adjacent to the trigger detecting period is larger than a difference in peak values of the waveform part based on which the reception timing is specified with respect to waveform parts at front and back periods adjacent to the reception timing specifying period.