PURPOSE: To sharpen the rise characteristic of amplitude and increase frequency band utilization rate by inserting a periodic delay circuit to at least one of the arbitrary positions of two optical guides of different electrical lengths connected by a polarizer and analyzer.

CONSTITUTION: At the time when the light containing central frequencies f1,... f2n enters from an incident light guide 4, the basic wave guiding sequence of said light and the action of each constituting circuit are identical to those of known ones. In this invention, the phase of the light emitted from a double refractive crystal 3 and entered a periodic delay circuit 7 causes a delay in the phase at fixed period intervals in the delay circuit 7 and receives the stepwise change in the phase of the light. Because of this, the phase difference of the light having propagated in the two optical guides becomes integer times of π of several times in one channel frequency interval of the branching filter 1 and therefore the phase difference may be made close to the value of π flatly over a wide frequency band. Because of this, at the time when the light having propagated in the two optical guides 5, 6 is going to be combined in an analyzer 2, it becomes of extremely sharp rise.