An optical unit for a wavelength-selecting switch according to the present invention comprises: an input port; a dispersion section that produces wavelength dispersion of input light that is input from the input port; a light-collecting element that collects the light dispersed by the dispersion section; an output port; an optical path correction section that shifts the light that is dispersed by the dispersion section; an adjustment section that changes the amount of shift produced by the optical path correction section; and a casing that hermetically seals the input port, dispersion section, light-collecting element, output port, and optical path correction section. The casing has an optically transparent section in a location onto which the light that is collected by the light-collecting element is directed. The adjustment section is arranged outside the casing. The optical path correction section can be controlled from outside the casing by the adjustment section.