Periodic displacement occurs in body tissue due to heartbeat. A peak level D of the movement distance of the body tissue is detected (Step 21), and a heartbeat cycle T is calculated from a frequency spectrum (Steps 22 and 23). By dividing twice the peak level D by the heartbeat cycle T, the moving velocity of the body tissue in a unit heartbeat cycle is calculated (Step 24). By dividing the moving velocity by a frame rate r, an average movement distance of the body tissue between frames is calculated (Step 25). In a case where the average movement distance is smaller than a predetermined threshold value, a time interval between the frames used for the calculation of the movement distance is extended (being Step 26 NO, Step 27).