A cavity is defined inside an injection mold to provide a contour of a light guide body. Also provided in the injection mold is a through hole communicating with the cavity and allowing an optical fiber to be inserted in the through hole. The through hole includes a columnar space provided in at least a portion contiguous to the cavity. The columnar space has a predetermined length and a cross section similar to that of the optical fiber, and a clearance left between the columnar space and the optical fiber when the optical fiber is inserted in the through hole is 30 .mu.m or less.