An optical diode. A cholesteric liquid crystal (CLC) layer has a selective reflection wavelength band with a left-handed helical structure, and a phase shifter for changing a phase difference between two intrinsic polarized light components of left-handed circularly polarized light having a wavelength within the selective reflection wavelength band of the CLC layer. When left-handed circularly polarized light having a wavelength within the selective reflection wavelength band of the CLC layer is incident on the phase shifter in the optical diode, the phase shifter turns the left-handed circularly polarized light into right-handed circularly polarized light, which can be transmitted through the CLC layer. When left-handed circularly polarized light having a wavelength within the selective reflection wavelength band of the CLC layer is incident on the CLC layer, it is selectively reflected by the CLC layer.