An optical deflection element for splitting light rays in accordance with a
wavelength of incident light, includes a photonic crystal having an
entrance end face and an exit end face, and having a refractive index
which periodically changes depending upon positions thereof. The optical
deflection element includes a region in which a propagation angle
.theta..sub.C of the incident light within the photonic crystal changes in
accordance with the wavelength of the incident light having entered at an
incident angle .theta..sub.IN into the entrance end face, and a
relationship among a wavelength .lambda. of the incident light in vacuum,
a lattice constant A of the photonic crystal, the incident angle
.theta..sub.IN and the propagation angle .theta..sub.C satisfies
(.differential..theta..sub.C
/.differential.(A/.lambda.))/(.differential..theta..sub.C
/.differential..theta..sub.IN)>10.
