The head end surface of an optical fiber (2) and the head end surface of an optical fiber (12) are butted to each other in an alignment member (22) of an adapter (21). In the butting, because one of the members clasping the optical fiber (2) is a plate (4a) that is short, the optical fiber (2) is bent and buckled between the left end of the short plate and the right end of the alignment member. Members clasping the optical fiber (12) are a long plate (14b) and a V-grooved base plate (13), and the optical fiber (12) is therefore horizontally fixed between each of right ends of the long plate and of the V-grooved base plate (13) and the left end of the alignment member. This results that the head end surface of the optical fiber (2) and the head end surface of the optical fiber (12) are in tight contact with each other, achieving excellent optical transmission between both optical fibers.