An optical amplifier comprising: an amplifying optical fiber including: a plurality of cores to which an active element is doped; a first cladding enclosing the plurality of cores, wherein no gap is situated between the first cladding and the cores; and a second cladding enclosing the first cladding; a pumping light source configured to emit pumping light for pumping the active element; a plurality of single core optical fibers to which communication light is entered; and an optical synthesizer configured to enter communication light incident on cores of the plurality of single core optical fibers and thereafter to an end face of a core of the amplifying optical fiber corresponding to the core of one of the plurality of single core optical fibers and to enter pumping light emitted from the pumping light source to a predetermined position on one end face of the amplifying optical fiber, wherein: the plurality of cores is disposed around a center axis of the first cladding; the first cladding has a two-layer structure formed of a solid inner layer passed through the center axis of the first cladding and an outer layer enclosing the inner layer and the plurality of cores, wherein no gap is situated between the outer layer and the cores; a refractive index of the core is provided higher than refractive indexes of the inner layer and the outer layer; the refractive indexes of the inner layer and the outer layer are provided higher than a refractive index of the second cladding; the refractive index of the inner layer is provided lower than the refractive index of the outer layer; and the optical synthesizer enters the pumping light to an area other than the center axis of the first cladding on the one end face of the first cladding. a plurality of cores to which an active element is doped; a first cladding enclosing the plurality of cores, wherein no gap is situated between the first cladding and the cores; and a second cladding enclosing the first cladding; a first cladding enclosing the plurality of cores, wherein no gap is situated between the first cladding and the cores; and a second cladding enclosing the first cladding; a pumping light source configured to emit pumping light for pumping the active element; a plurality of single core optical fibers to which communication light is entered; and an optical synthesizer configured to enter communication light incident on cores of the plurality of single core optical fibers and thereafter to an end face of a core of the amplifying optical fiber corresponding to the core of one of the plurality of single core optical fibers and to enter pumping light emitted from the pumping light source to a predetermined position on one end face of the amplifying optical fiber, wherein: the plurality of cores is disposed around a center axis of the first cladding; the first cladding has a two-layer structure formed of a solid inner layer passed through the center axis of the first cladding and an outer layer enclosing the inner layer and the plurality of cores, wherein no gap is situated between the outer layer and the cores; a refractive index of the core is provided higher than refractive indexes of the inner layer and the outer layer; the refractive indexes of the inner layer and the outer layer are provided higher than a refractive index of the second cladding; the refractive index of the inner layer is provided lower than the refractive index of the outer layer; and the optical synthesizer enters the pumping light to an area other than the center axis of the first cladding on the one end face of the first cladding. the plurality of cores is disposed around a center axis of the first cladding; the first cladding has a two-layer structure formed of a solid inner layer passed through the center axis of the first cladding and an outer layer enclosing the inner layer and the plurality of cores, wherein no gap is situated between the outer layer and the cores; a refractive index of the core is provided higher than refractive indexes of the inner layer and the outer layer; the refractive indexes of the inner layer and the outer layer are provided higher than a refractive index of the second cladding; the refractive index of the inner layer is provided lower than the refractive index of the outer layer; and the optical synthesizer enters the pumping light to an area other than the center axis of the first cladding on the one end face of the first cladding.