A chirp switching circuit comprises a Mach-Zehnder modulator having a Y-branched part for branching an incoming optical signal into first and second optical signals and an X-branched part merging the first and second optical signals with each other, the Mach-Zehnder modulator causing phase modulation in the first and second optical signals by a modulation signal, and a directional coupling optical switch that switches first and second optical output signals output from the X-branched part of the Mach-Zehnder modulator by merging the first and second output optical signals in response to a chirp switching control signal.