An optical switch is constituted of a wavelength filter and a control device. The wavelength filter includes a first waveguide exhibiting a transmission band of a predetermined basic mode and a second waveguide, arranged in at least one location of the first waveguide, exhibiting a transmission band with a cutoff frequency corresponding to a finite value included in the transmission band of the basic mode. A pair of optical couplers constituting a Mach-Zehnder interferometer is connected to opposite ends of a filter unit including the first waveguide and the second waveguide. When a plurality of wavelength filters is cascaded, these wavelength filters differ from each other in terms of the cutoff frequency of the second waveguide. The control device changes the cutoff frequency of the second waveguide within the transmission band of the first waveguide, thus adjusting the filtering band of the wavelength filter.