An optical diode which can lower the cost and save electric power is provided. An optical diode 21 of the present invention comprises a cholesteric liquid crystal (CLC) layer 2 having a selective reflection wavelength band with a left-handed helical structure, and a phase shifter 24 for changing the 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 2. When left-handed circularly polarized light having a wavelength within the selective reflection wavelength band of the CLC layer 2 is incident on the phase shifter 24 in the optical diode 21, for example, the phase shifter 24 turns the left-handed circularly polarized light into right-handed circularly polarized light, this right-handed circularly polarized light can be transmitted through the CLC layer 2. When left-handed circularly polarized light having a wavelength within the selective reflection wavelength band of the CLC layer 2 is incident on the CLC layer 2, on the other hand, this left-handed circularly polarized light is selectively reflected by the CLC layer 2. Thus, an optical diode characteristic is realized. This optical diode 21 does not require a large amount of time and labor to manufacture. Further, since no nonlinear optical effect is utilized, it is not necessary for laser light having such a high intensity as to cause the nonlinear optical effect to be incident as incident light.