An optical isolation device comprising at least one optical isolation element, wherein the optical isolation element comprises a polarization conversion element and a polarizer, which are sequentially disposed, wherein the polarization conversion element is configured to convert unpolarized incident light comprising first and second polarized lights, which are an orthogonal mode pair to each other, into one kind of polarized light and to output it, wherein the polarization conversion element comprises a first holographic optical element, a second holographic optical element and a waveguide medium in this order, wherein each of the first holographic optical element, the second holographic optical element and the waveguide medium has a light entrance surface and a light exit surface, wherein the first holographic optical element is an optical element configured to output the unpolarized incident light incident on its light entrance surface at an incident angle that is a first angle which is an angle of greater than -90 degrees and less than 90 degrees, to an output angle that is a second angle which differs from the first angle and is in a range of greater than 0 degrees and less than 90 degrees or in a range of greater than -90 degrees and less than 0 degrees, wherein the second holographic optical element is an optical element configured to output, among the unpolarized light incident on it at an incident angle that is a third angle, the first polarized light with an output angle that is a fourth angle and the second polarized light with an output angle that is a fifth angle which differs from the fourth angle, wherein the waveguide medium comprises a sidewall provided with a retarder and is formed such that the first polarized light having the output angle of the fourth angle and being incident on the waveguide medium to be output from its light exit surface without being incident on the sidewall provided with the retarder or after being incident thereon even times and totally reflected, and so as for the second polarized light having the output angle of the fifth angle and being incident on the waveguide medium to be output from its light exit surface after being incident on the sidewall provided with the retarder odd times and totally reflected, wherein the sum of the second angle and the third angle is in a range of 80 degrees to 100 degrees, and wherein the sum of the fourth angle and the fifth angle is in a range of -10 degrees to 10 degrees. wherein the optical isolation element comprises a polarization conversion element and a polarizer, which are sequentially disposed, wherein the polarization conversion element is configured to convert unpolarized incident light comprising first and second polarized lights, which are an orthogonal mode pair to each other, into one kind of polarized light and to output it, wherein the polarization conversion element comprises a first holographic optical element, a second holographic optical element and a waveguide medium in this order, wherein each of the first holographic optical element, the second holographic optical element and the waveguide medium has a light entrance surface and a light exit surface, wherein the first holographic optical element is an optical element configured to output the unpolarized incident light incident on its light entrance surface at an incident angle that is a first angle which is an angle of greater than -90 degrees and less than 90 degrees, to an output angle that is a second angle which differs from the first angle and is in a range of greater than 0 degrees and less than 90 degrees or in a range of greater than -90 degrees and less than 0 degrees, wherein the second holographic optical element is an optical element configured to output, among the unpolarized light incident on it at an incident angle that is a third angle, the first polarized light with an output angle that is a fourth angle and the second polarized light with an output angle that is a fifth angle which differs from the fourth angle, wherein the waveguide medium comprises a sidewall provided with a retarder and is formed such that the first polarized light having the output angle of the fourth angle and being incident on the waveguide medium to be output from its light exit surface without being incident on the sidewall provided with the retarder or after being incident thereon even times and totally reflected, and so as for the second polarized light having the output angle of the fifth angle and being incident on the waveguide medium to be output from its light exit surface after being incident on the sidewall provided with the retarder odd times and totally reflected, wherein the sum of the second angle and the third angle is in a range of 80 degrees to 100 degrees, and wherein the sum of the fourth angle and the fifth angle is in a range of -10 degrees to 10 degrees.