In an in-line optical isolator, a first polarization separation element 91, a Faraday rotator 6 made of a BIG film, and a second polarization separation element 92 are placed in that order. The isolator further includes a first optical fiber collimator 1a being placed at the forward-beam incident side of the first polarization separation element 91 and including a collimating lens 101 and a first optical fiber 31, and a second optical fiber collimator 2a being placed at the forward-beam exit side of the second polarization separation element 92 and including a collimating lens 102 and a second optical fiber 32 connected to a fiber amplifier. Also, an edge filter 100, which transmits light emitted from the first optical fiber 31 and having a wavelength equal to an oscillation wavelength and which reflects light emitted from the first optical fiber 31 and having wavelengths shorter than this wavelength, is placed between the second optical fiber collimator 2a and the second polarization separation element 92.