1. An ultrasonic diagnostic apparatus comprising: an ultrasonic probe configured to acquire a plurality of ultrasonic data by repeatedly scanning a two dimensional section in a longitudinal direction of a target blood vessel with an ultrasonic wave; an image generation processor configured to generate a plurality of ultrasonic images by using the plurality of ultrasonic data; and a measurement processor configured to extract a measurement region based on one of a pixel value and a rate of change in pixel value from each of the ultrasonic images, calculate lateral widths of the measurement region at at least two positions, determine an ultrasonic image having a smallest rate of change in pixel value from the plurality of ultrasonic images based on the lateral widths, and measure an intima media thickness of the target blood vessel by using the ultrasonic image having a smallest rate of change in pixel value.