1. An ultrasonic diagnostic apparatus, comprising: an ultrasonic scanner 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; interest region setting circuitry configured to set a three-dimensional region of interest of the tissue of the subject body for the volume data at a predetermined time; motion vector information generating circuitry configured to generate three-dimensional motion vector information on the region of interest at times other than the predetermined time by processing using pattern matching; gauge setting circuitry configured to set a reference surface in a three-dimensional space at the predetermined time, set at least one three-dimensional position of a strain gauge on the reference surface, and set the at least one three-dimensional position of the strain gauge at times other than the predetermined time, using the three-dimensional motion vector information on the region of interest, the strain gauge being a line segment on the tissue of the subject body and extending in a wall thickness direction of heart tissue; image generating circuitry configured to generate a three-dimensional strain gauge image depicting the at least one three-dimensional position of the strain gauge and the position of the reference surface in the three-dimensional space in a three-dimensional manner; and a display that displays the three-dimensional strain gauge image.