A conventional medical image processing apparatus is capable of forming a section perpendicular to a reference plane, but has a drawback that a section intersecting the reference plane at a desired angle other than the right angle cannot be formed. A sectional line is defined in a three-dimensional image viewed perpendicularly to a reference plane. Then, the displayed image is switched to an image viewed along the sectional line, and a line indicative of the reference plane and a line indicative of a sectional plane based on the sectional line are displayed in the image. In turn, the line indicative of the displayed sectional plane is inclined with respect to the line indicative of the reference plane. Then, a sectional image taken along the sectional plane defined by the inclined line is displayed.