A capillary tube holds an optical fiber. The capillary tube includes an insertion hole for inserting and fixing the optical fiber formed therein, a cylindrical surface as an outer circumferential surface, and a groove formed in the cylindrical surface in an axial direction thereof. Regions from the cylindrical surface to both inner side surfaces of the groove each have a projecting curved surface, and regions from both the inner side surfaces of the groove to a bottom of the groove each have a recessed curved surface.