To obtain the light isolator optical system which has minimum insertion loss by specifying the effective optical path from the incidence/projection end of an optical fiber to the other opening end of a light isolator.

A light isolator 1 is arranged near the optical fiber 9 and the distance from the incidence/projection end B of the optical fiber 9 to the opening end A of the light isolator 1 on the opposite side from the optical fiber 9 is denoted as L. In this case, L is the effective optical path length and a value obtained by dividing the length by a refractive index is used. Further, NA=sinθ holes, where NA is the numerical aperture of the optical fiber 9 and θ is the angle of a beam which is made incident on the optical fiber 9 or projected from the optical fiber 9. At this time, the light isolator 1 is positioned within a range of the distance L shown by the equation so that the beam 10 which is made incident on or projected from the optical fiber 9 is not eclipsed with the aperture diameter (d) of the light isolator.