To provide a measuring device capable of measuring easily a tubular measuring object.

This device includes an irradiation means 4 for irradiating light over the whole circumference of an inner circumferential surface 2a of the tubular measuring object 2, a projection means 6 for projecting reflected light of light irradiated by the irradiation means 4, and an imaging means 7 for imaging an image projected by the projection means 6. The irradiation means 4 and the imaging means 7 are arranged oppositely, and the projection means 6 is arranged between the irradiation means 4 and the imaging means 7, and the irradiation means 4 has a constitution for irradiating light to the inner circumferential surface 2a extendedly conically so as to form linear ring light L between the irradiation means 4 and the imaging means 7 on the inner circumferential surface 2a, and the ring light L is projected to the projection means 6, and the ring light projected to the projection means is imaged by the imaging means 7, to thereby measure the measuring object 2.