An ultrasound beam is repeatedly transmitted and received to and from an object, so that an echo signal is repeatedly obtained. A velocity of each of a plurality of portions on the ultrasound beam is calculated based on the echo signal. A position of at least one specific portion of the plurality of portions is tracked in as it changes with the passage of time based on the velocity to obtain a path where the specific portion moves. Conventionally the change of brightness of the M-mode image was tracked to obtain the path. According to the present invention, since the path is obtained based on the velocity having high resolution, the accuracy of track can be improved.