A transparent tube is placed in a parallel laser light beam emitted from a light projector and reaching a photoreciever, and a detection signal indicating an amount of light received corresponding to a width direction position of the parallel laser light beam is obtained from the photoreceiver. Peaks formed in the detection signal by beams reflected by an inner circumferential surface of the transparent tube and incident to the photoreceiver are detected; width direction positions of two intersection points where the peaks cross a predetermined threshold value are detected; the width direction positions of the light beam reflected by the inner circumferential surface of the transparent tube are calculated from an average value of the two intersection points; and an internal diameter of the transparent tube is measured from the width direction positions.