PURPOSE: To constitute the apparatus so as to be able to surely detect existence of occurrence of deviated wall thickness of a protective resin applied to the surface of an optical fiber without an individual difference and directly catch occurrence of deviated wall thickness without always watching projected image on a screen.

CONSTITUTION: Projected images on screens 4a and 4b of an optical fiber core wire 2 are picked up by image pickup means 6a and 6b and the distances L1 and L2 from the center positions of the projected images to lateral final positions are obtained by a deviation operating means 16 based on these picked-up image data and the existence of deviated wall thickness is judged by a judging means 16 based on the values of both distances L1 and L2. Then, the occurrence of deviated thickness is alarmed by an alarm means 18 in response to deviated wall thickness abnormality signal outputted when it is judged to be deviated wall thickness by the judging means 16.