It is an object of this invention to overcome the problems of prior art by a unique arrangement based on a novel idea, and to provide an optical device having desired characteristics and an optical module and optical system incorporating the optical device. Provided is a photonic crystal including a three-dimensional periodic structure formed by using a first material which changes in properties relatively easily and a second material which does not change in properties relatively easily, wherein the first material is preferentially allowed to change in properties to make a refractive index difference between the first and second materials larger than that before the property change. Also provided is an optical device including a photonic waveguide, gain means, and reflecting means, wherein resonance is generated by reversing the propagating direction of light propagating in the photonic waveguide by the reflecting means while a gain is given to the light by the gain means.