A bridge fiber includes a core layer and an outer layer which has an index of refraction higher than that of the core layer and covers the outer peripheral surface of the core layer. The outer layer is surrounded by a substance such as the atmosphere having an index of refraction lower than an index of refraction n2 of the outer layer. An area AR1 of the outer layer at one end face of the bridge fiber is an area that is to be optically coupled to an end face of a core of each of a plurality of pumping light inputting optical fibers, while an area AR2 of the core layer at another end face of the bridge fiber is an area that is to be optically coupled to an end face of a core of an amplification optical fiber.