To provide an improve switchable filter by alternately arranging a phase shifter on at least one arm and reflection filters on respective arms on the middle section of the filter.

The switchable filter consists of an optical demultiplexer connected to an input waveguide, one or more output waveguides and plural interference waveguides. The switchable optical filter 10 constituted so as to be used as a gain equalization filter is provided with a Mach-Zchender interferometer 11 consisting of a pair of waveguide arms 12, 13 prolonging between a pair of couplers 14, 15. The middle section (arm part between couplers 14, 15) of the Mach-Zchender interferometer 11 contains alternately plural phase shifters 1, 2,..., N, and passive brag gratings Λ1, Λ2,..., ΛN, ΛN+1. The filter is provided with reflection ports A, B and transmission ports C, D.