1. An ultrasonic diagnostic device comprising: a volume data formation unit for transmitting and receiving an ultrasonic wave to and from a three-dimensional space containing a target tissue to form volume data composed of a plurality of sets of voxel data; a reference line setting unit for setting a first reference line passing through the target tissue based on a plurality of planes set within the three-dimensional space and which intersect each other; a boundary finding unit for finding a boundary of the target tissue by referring to voxel data along the first reference line and a second reference line intersecting the first reference line; a shape parameter calculator unit for calculating a plurality of shape parameters for specifying a three-dimensional region of interest related to the target tissue based on the found boundary; and a region-of-interest setting unit for setting the region of interest within the three-dimensional space based on the plurality of shape parameters.