1. An ultrasonic diagnostic apparatus comprising: a data collection unit that collects volume data over one or more periods of movement of tissue of a subject body, which moves periodically, by ultrasonically scanning the tissue of the subject body; an interest region setting unit that sets a three-dimensional region of interest of the tissue of the subject body for the volume data in a predetermined time phase; a motion vector information generating unit that generates motion vector information on the region of interest in time phases other than the predetermined time phase by processing using pattern matching; a gauge setting unit that sets at least one three-dimensional position of a strain gauge, which is a line segment on the tissue of the subject body, using the motion vector information on the region of interest; an image generating unit that generates a three-dimensional strain gauge image obtained by imaging the set strain gauge in a three-dimensional manner; and a display unit that displays the three-dimensional strain gauge image in a predetermined form.