Method and apparatus for measuring speed-of-sound

A speed-of-sound measurement apparatus includes a wave transmission module (21) for transmitting an ultrasonic wave to a front surface of a subject's body, a plurality of wave reception modules (22) which each receives the ultrasonic wave from the subject's body and outputs a waveform signal corresponding to the received ultrasonic wave, a presumed propagation time calculating module (51) for calculating a propagation time from when the ultrasonic wave is transmitted by the wave transmission module to when the ultrasonic wave arrives at each of the wave reception modules after propagating along the front surface of the subject's body or inside the subject's body, based on a presumed value of speed-of-sound in the subject's body and a front surface shape of the subject's body, a validity index value calculating module (52) for finding a validity index value to be an index of validity of the propagation time based on the waveform signals outputted by at least two of the plurality of wave reception modules, and a speed-of-sound deriving module (53) for finding the speed-of-sound in the subject's body based on the validity index value.