PURPOSE:To decrease the number of optical components and to reduce the cost by constituting a polarized wave coupler and a multiplexer integrally as a prism unit. CONSTITUTION:A total reflecting film 34 is formed on one surface of a 1st prism 33 which has a 45 deg. included angle and a multiplexing film 35 is formed on the other surface. On the latter surface of the 1st prism 33, a quadrangular prism 36 which has a 2nd included angle of 45 deg. opposite a 1st included angle is stuck so that the 1st included angle is enough to multiplex signal light and pumping light sufficiently and adjoins to the included angle of the 1st prism 33. A polarized wave multiplexing film 38 is formed on the surface of the quadrangular prism 36 facing the surface where a multiplexing film 35 is formed and a triangular prism 40 is stuck on the surface of the quadrangular prism 36 where the polarized wave multiplexing film 38 is formed to constitute the prism 32, thus constituting the polarized wave coupler and multiplexer in one body. Consequently, the constitution of the optical coupler can be simplified.