1. An ultrasound diagnostic apparatus, comprising: a data acquisition section which performs transmission and reception of ultrasound with respect to a three-dimensional space including an object tissue to acquire volume data; a representative cross section processing section which, when a plurality of manual tracing lines are formed on a plurality of representative cross sections which are set with regard to the object tissue, applies a first correction processing to each manual tracing line; a non-representative cross section processing section which forms, by interpolation processing based on a plurality of manual tracing lines to which the first correction processing has been applied, a plurality of interpolation tracing lines on a plurality of non-representative cross sections which are set with regard to the object tissue, and applies a second correction processing to each interpolation tracing line; and a unit which extracts object tissue data from the volume data or calculates a volume of the object tissue, based on rows of tracing lines formed of a plurality of manual tracing lines to which the first correction processing has been applied and a plurality of interpolation tracing lines to which the second correction processing has been applied.