An optical amplifier comprising: an amplification multi-core optical fiber having: a plurality of first cores each extending along a first central axis and comprised of silica glass doped with a rare earth element; and a first cladding surrounding each of the plurality of first cores and comprised of silica glass; a coupling multi-core optical fiber having: a plurality of second cores each extending along a second central axis and optically connected to any one of the plurality of first cores, the plurality of second cores each comprised of silica glass; and a second cladding surrounding each of the plurality of second cores and comprised of silica glass, the coupling multi-core optical fiber having a mode-coupling coefficient of 1 [1/m] or more at a predetermined pump wavelength capable of pumping the rare earth element; and a pumping light source optically connected to at least one second core among the plurality of second cores and configured to supply pumping light of the predetermined pump wavelength to each of the plurality of first cores through the coupling multi-core optical fiber, characterized by that the pumping light from the pumping light source, coupled to the at least one second core, is coupled from the at least one second core to each remaining second core except the at least one second core, and then coupled from each of the plurality of second cores to an optically connected first core among the plurality of first cores, wherein each of the plurality of second cores has a core diameter of 4 to 16 μm and an interval between adjacent ones of the second cores is equal to the core diameter or more and equal to 30 μm or less. an amplification multi-core optical fiber having: a plurality of first cores each extending along a first central axis and comprised of silica glass doped with a rare earth element; and a first cladding surrounding each of the plurality of first cores and comprised of silica glass; a coupling multi-core optical fiber having: a plurality of second cores each extending along a second central axis and optically connected to any one of the plurality of first cores, the plurality of second cores each comprised of silica glass; and a second cladding surrounding each of the plurality of second cores and comprised of silica glass, the coupling multi-core optical fiber having a mode-coupling coefficient of 1 [1/m] or more at a predetermined pump wavelength capable of pumping the rare earth element; and a pumping light source optically connected to at least one second core among the plurality of second cores and configured to supply pumping light of the predetermined pump wavelength to each of the plurality of first cores through the coupling multi-core optical fiber, characterized by that the pumping light from the pumping light source, coupled to the at least one second core, is coupled from the at least one second core to each remaining second core except the at least one second core, and then coupled from each of the plurality of second cores to an optically connected first core among the plurality of first cores, wherein each of the plurality of second cores has a core diameter of 4 to 16 μm and an interval between adjacent ones of the second cores is equal to the core diameter or more and equal to 30 μm or less.