An optical wavelength switch having a planar wave-guide formed on a substrate including a wave-guide-type diffraction grating having an input/output wave-guide with an under-clad layer on a sacrificial layer formed on the substrate, a core layer formed on the under-clad layer and an over-clad layer formed on the core layer. The switch including a first slab wave-guide connected with the input/output wave-guide, an array wave-guide with one side connected to the first slab wave guide, a second slab wave-guide connected to the other side of the array wave-guide, and a movable girder secured to the substrate. The movable girder has the same under-clad layer, core layer and over-clad layer as the wave-guide-type diffraction grating. The switch has a minor at the tip of the movable girder, the mirror facing an end face of the second slab wave-guide, and the displaceable along a direction perpendicular to the optical axis.