PURPOSE: To correctly position an optical fiber connector with a plug or without a plug and fix same by axially projecting the central gripping portion of a collet chuck portion and storing the forward end portion of the plug in the outer peripheral space of the projecting portion.

CONSTITUTION: A working chuck 1 is assembled by a spindle 4 incorporated in the interiors of housings 2, 3 and a collet chuck 5. This spindle 4 is cylindri cal and supported in such a manner as to freely rotate with an input gear 7 by two bearings 6 inside the housings 2, 3. The collet chuck 5 is inserted in such a manner as to freely slide concentrically and axially in the interior of the spindle 4, and a taper surface 11 of the outer peripheral portion of the forward end thereof comes into contact with a taper surface 12 of the spindle 4 with the same gradient. The collet chuck can be deformed in the radial direc tion by four axial expanding slots 13, so that an optical fiber connector with plug or without a plug is held by a gripping portion 14.