An optical device which includes a GI-type photonic crystal slab which includes: a first member which has a distribution of refractive indexes reduced in both directions from an optical axis of incident light as to a first direction vertical to the optical axis; and a second member periodically placed in substance among the first members as to a second direction different from the first direction, wherein the distribution of refractive indexes of the first member which relates to the first direction, a thickness which relates to the first direction of the GI-type photonic crystal slab, a wavelength of the incident light and an incident end beam spot radius .omega..sub.1 which relates to the first direction inside an incident end of the GI-type photonic crystal slab entered by the light of the incident light are determined to have the incident light substantially confined inside the GI-type photonic crystal slab as to the first direction.