A wavelength-selective switch with a switch unit having a lens array, a first lens and a beam expander, and a branching unit having a second lens, a diffraction grating, a third lens and a mirror, in which the switch unit and the branching unit are accommodated in independent casings, and a confocal point of the light transmitted through the first lens and the light transmitted through the second lens are arranged on a connecting surface of the casing of the switch unit and the casing of the branching unit.