The average velocity of a tissue is calculated for each frame in accordance with compression/release, and a reference waveform is generated by using the calculated velocity. A time phase in which the average velocity becomes 0 (i.e., a stationary time phase of the tissue) is specified by the reference waveform. In strain computation, time integration is performed for strain computation in a compression period or a release period with reference to the specified stationary time phase. This can properly and automatically visualize how the contraction of the tissue is accumulated from the start of compression (i.e., a time phase corresponding to V = 0+) or the contraction of the tissue becomes maximum in a compression end time phase (i.e., a time phase corresponding to V = 0-) in a compression period and how the tissue expands from the start of release (i.e., a time phase corresponding to V = 0-) or the expansion of the tissue becomes maximum in a release end time phase (i.e., a time phase corresponding to V = 0+) in a release time phase.
