An optical isolator has a high ratio of the quantity of signals S to that of noises N (S/N ratio) and can be miniaturized such a size that it can easily be incorporated into a laser module. The optical isolator 1 according to the present invention is one to be connected to a semiconductor laser 11 having a linear polarization plane and comprises a Faraday's rotator 5 and a magnetic material 4 for establishing a magnetic field around the rotor on the light-incident side of the isolator and an optical material 6 for polarization on the light-outputting side thereof. In particular, the optical isolator 1 of the present invention is one to be connected to a semiconductor laser 11 having a linear polarization plane while aligning a polarization direction guide 3 of the isolator with the semiconductor laser 11, comprises a Faraday's rotator 5 whose light-incident and -outputting sides are desirably inclined with respect to the optical path and an annular magnetic material 4 surrounding the rotator 5, on the light-incident side of the isolator and an optical material 6 for polarization, on the light-outputting side thereof, whose light-incident and -outputting sides are desirably inclined with respect to the optical path, and the optical material for polarization is more desirably made of a metal-containing glass material.