Biological tissues periodically change their position with the heartbeat. In the present invention, a peak level D of the amount of movement of a biological tissue is detected (Step 21) and the cardiac cycle T is calculated from a frequency spectrum (Steps 22, 23). The peak level D is multiplied by 2 and divided by the cardiac cycle T in order to calculate the moving speed per unit cardiac cycle of the biological tissue (Step 24). The moving speed is divided by a frame rate r in order to calculate the average movement amount of the frames of the biological tissue (Step 25). If the average movement amount is less than a threshold value, the time interval between frames which is used to calculate the movement amount is increased (No in Step 26, Step 27).
