PROBLEM TO BE SOLVED: To provide an inside surface shape measurement instrument which can project a flat light beam and has such a structure that the instrument can be manufactured at low costs and is easily miniaturized.SOLUTION: An inside surface shape measurement instrument 1 for measuring an inside surface shape of a specimen by using an optical cutting method comprises: a projection unit 20 which includes a cylindrical outer peripheral part 23 having one or more slits extending in a circumferential direction and allowing light to pass therethrough, and a light source unit 22 having one or more light emitting elements and arranged in the outer peripheral part 23 and projects a light beam B having a prescribed thickness, on the specimen from the slits； and an imaging unit 30 which images the inside surface of the specimen on which the light beam is projected.【課題】平板状光束を投影することが可能でありながら、安価に製造でき、小型化が容易な構造を有する内面形状測定装置を提供する。【解決手段】光切断法を用いて被検物の内面形状を測定する内面形状測定装置１は、周方向に延びて光が透過可能な一つ以上のスリットを有する筒状の外周部２３と、一つ以上の発光素子を有して外周部内に配置された光源ユニット２２とを有し、被検物に対してスリットから所定の厚みの光束Ｂを投影する投影部２０と、光束が投影された被検物の内面を撮像する撮像部３０とを備えることを特徴とする。【選択図】図２
