To measure the volume of an ellipsoid easily and with high reliability.

In the step S31, a cursor is displayed at the middle point in a first axis of a tomograph calculated by a middle point calculating part as the initial position. In the step S32, the cursor is moved, and in the step S33, the starting point of a second axis is determined. Then, in the step S34, the cursor is displayed at the middle point in the first axis of the tomograph as the initial position. In the step S35, the cursor is moved, then in the step S36, the terminal point is determined and the processing is completed. The cursor in the steps S32 and S35 is controlled to move only in the direction perpendicular to the first axis passing the middle point calculated by the middle point calculating part by a movement direction control part of a cursor control part of the CPU.