PURPOSE: To obtain the coupling structure of optical fibers which allows the optical axes of these optical fibers to coincide with each other automatically by providing the structure with a means mechanically holding positions on which respective end surfaces of two optical fibers are opposed and liquid arranged so as to dip the opposed parts in the liquid and having large surface tension.

CONSTITUTION: Two optical fibers 1, 2 are arranged on a board 3 on which a V-shaped groove is formed so that respective end surfaces of the optical fibers 1, 2 are opposed each other and are mechanically held by pressing plates 4. Under the state that mercury 6 is not arranged, the end parts of the optical fibers 1, 2 are deviated at their axes. When the mercury 6 is dropped on a part on which the optical fibers 1, 2 are opposed and the end parts of both the optical fibers are dipped in the mercury 6, the optical fibers 1, 2 are bent and the center axes of the opposed end surfaces are automatically made to coincide with each other. Since the surface tension of the mercury is large, the slight interval between the optical fibers 1, 2 is kept at a space without inflow of the mercury. On the other hand, the surface tension is acted to change the shape of the surface so that the surface area is minimized.