Abstract: An inspection device is a device for inspecting an inner peripheral surface of an inspection target having a cylindrical shape. The device includes: an inspection part for emitting a terahertz wave to the inner peripheral surface and detecting a reflection wave of the terahertz wave; and a distance measurement part for measuring a distance between the inspection part and the inner peripheral surface. The inspection part and the distance measurement part are rotated along a circumferential direction of the inspection target. The axial-directional position of the inspection part relative to the inspection target is adjusted. The position of the inspection part on a plane which intersects with an axial direction of the inspection target is adjustable. The position adjustment mechanism is controlled such that a rotational center of the rotation part is positioned on a center axis of the inspection target on the basis of a measurement result of the distance measurement part.