PURPOSE: To measure the straightness of the small-diameter hole of a material to be measured to the deep part.

CONSTITUTION: A taget plate 11 having a hole 12 at the center is attached to a front end part 4 of a cylinder 4. Frames 6, which are outputted and inputted in the radial direction, are provided at the front end part and the rear end part of the cylinder 4. Cone-shaped pistons 8a and 8b having tapered parts 9 at the tips are contained in the cylinder 4. The frames 6 are made to project by pushing the front and rear cone-shaped pistons 8a and 8b with a spring 10. Thus, the cylinder 4 is aligned in a deep hole 2 of a material to be measured 1. An optical fiber 14 is made to pass in the cylinder 4, and light is emitted through the central hole 12. The position of the central hole 12 is observed with a microalignment telescope.