Inner-wall measuring instrument and offset-amount calculation method

An offset-amount calculation method for calculating an offset amount between a touch probe disposed at a first position of a base, which is relatively movable in three axis directions orthogonal to one another with respect to a placement surface on which an object to be measured is placed, and an image probe disposed at a second position of the base, the method comprising:

• calculating a first offset amount in a direction parallel to the placement surface between a rotation axis and a movement axis of the image probe capable of imaging the object to be measured with the direction parallel to the placement surface being an imaging direction, the image probe being rotated by a rotational drive unit around the rotation axis extending in a direction perpendicular to the placement surface and moved by a linear drive unit along the movement axis extending in the imaging direction;
• calculating a second offset amount in the direction parallel to the placement surface between the touch probe and the rotation axis;
• and
• calculating an offset amount between the touch probe and the image probe on the basis of the calculated first and second offset amounts.