1. An ultrasonic diagnostic apparatus comprising: a data acquisition unit configured to acquire a plurality of spatially overlapping ultrasonic images or a plurality of temporarily consecutive ultrasonic images; a block extraction unit configured to extract, for each pixel, a block having a predetermined size and including each pixel as a central pixel for each of the plurality of ultrasonic images; a covariance calculation unit configured to calculate a covariance for each pixel of each of the plurality of ultrasonic images by using the block for each pixel; and a generating unit configured to generate a synthesized image using the plurality of ultrasonic images and to control a level of each pixel of the synthesized image based on a covariance of each pixel of each of the plurality of ultrasonic images.