A normal vector on a regression plane in a reference time phase in a three-dimensional space is defined with regard to a moving tissue typified by a myocardial wall. Orthogonal projection vectors on the regression plane at each vertex (Pij(t)) in each time phase are calculated by using the normal vector on the regression plane, and the angle defined by the orthogonal projection vectors is calculated, thereby acquiring a local rotational angle at each vertex (Pij(t)) in each time phase relative to the reference time phase.
