An optical isolator using a photonic crystal. The optical isolator includes an input portion that in turn includes an input waveguide part and an input waveguide having a taper portion formed in the input waveguide part. An output portion includes an output waveguide part and an output waveguide formed in the output waveguide part continuously with the input waveguide. The output waveguide includes a backward directional optical signal shield surface having an inclination greater than the taper portion with respect to an optical signal transmission center axis. The optical isolator is operable to have an optical signal substantially transmitted in a forward direction from the input portion to the output portion and further operable to substantially not transmit the optical signal in a backward direction from the output portion to the input portion.