1. An ultrasonic diagnostic apparatus, wherein a whole scan region is divided into a main scan region and a subscan region, the subscan region is divided into a plurality of small regions, the main scan region and the subscan region are, respectively, made up of a two-dimensional scan region or and a three-dimensional scan region, and each main scan of the main scan region is performed, in an alternating pattern, between a subscan of one small region of the subscan region and a subscan of another small region of the subscan region, wherein, in the whole scan region made up of the main scan region and the subscan region, the main scan region is main-scanned at equal time intervals regardless of where the main scan region is placed in the whole scan region, and wherein the whole scan region spreads to a two-dimensional or three-dimensional region and a plurality of scan regions exist.