1. An ultrasonic diagnostic apparatus comprising: an ultrasonic probe which generates an ultrasonic beam; a volume data set collecting unit which collects a plurality of volume data sets corresponding to a plurality of three-dimensional scan ranges via the ultrasonic probe, the plurality of three-dimensional scan ranges partly overlapping one another; a region of interest setting unit which sets, in accordance with a user instruction, a region of interest on a first tomogram generated from particular one of the plurality of volume data sets; a tomogram generating unit which generates the first tomogram from the particular volume data set and also generates a second tomogram associated with a section including the region of interest from another volume data set corresponding to the three-dimensional scan range including the region of interest; and a display unit which displays the first tomogram and the second tomogram.