A photonic crystal 2 with plasticity is arranged by making microspheres of silica or barium titanate or air bubbles be contained in a gel substance. When an external force is applied to this photonic crystal, photonic crystal 2 deforms and the photonic band gap is readily changed thereby. When the photonic band gap changes, the passage of light of a specific wavelength is restricted. Light of a desired wavelength is thus output from photonic crystal 2. With this invention, this wavelength can be varied readily by means of an external force.