An optical coupler (4, 4C, 6, 6A), comprising: a main optical fiber (4a, 4Ba, 6a) that includes a plurality of cores (2ba, 4aa, 4Baa, 6aa), each of which is configured to transmit signal light, an inner cladding portion (4ab, 4Bab, 6ab), formed around an outer periphery of the cores (2ba, 4aa, 4Baa, 6ab), the inner cladding portion allowing multi-mode light and having a refractive index lower than a refractive index of the cores (2ba, 4aa, 4Baa, 6aa), and an outer cladding portion (4ac, 6ac), formed around an outer periphery of the inner cladding portion (4ab, 4Bab, 6ab), having a refractive index lower than the refractive index of the inner cladding portion (4ab, 4Bab, 6ab), a part of the outer cladding portion (4ac, 6ac) of which in a longitudinal direction being removed; and at least one pumping light input-output optical fiber (4b, 6b) an end of which is fusion-spliced to the inner cladding portion (4ab, 4Bab, 6ab) of the main optical fiber (4a, 4Ba, 6a) at a portion where the outer cladding portion (4ac, 6ac) is removed, an average refractive index of which the pumping light input-output optical fiber in a contact region where the end of the pumping light input-output optical fiber (4b, 6b) is in contact with the inner cladding portion (4ab, 4Bab, 6ab) being larger than the refractive index of the inner cladding portion (4ab, 4Bab, 6ab), wherein the optical coupler is configured such that pumping light propagating in the pumping light input-output optical fiber (4b, 6b) is coupled to the side surface of the inner cladding portion (4ab, 4Bab, 6ab) from the contact region and propagates in the inner cladding portion (4ab, 4Bab, 6ab), or the pumping light propagating in the inner cladding portion (4ab, 4Bab, 6ab) is coupled from the side surface of the inner cladding portion to the contact region, and from the contact region to the pumping light input-output optical fiber (4b, 6b) and propagates in the pumping light input-output optical fiber (4b, 6b). a main optical fiber (4a, 4Ba, 6a) that includes a plurality of cores (2ba, 4aa, 4Baa, 6aa), each of which is configured to transmit signal light, an inner cladding portion (4ab, 4Bab, 6ab), formed around an outer periphery of the cores (2ba, 4aa, 4Baa, 6ab), the inner cladding portion allowing multi-mode light and having a refractive index lower than a refractive index of the cores (2ba, 4aa, 4Baa, 6aa), and an outer cladding portion (4ac, 6ac), formed around an outer periphery of the inner cladding portion (4ab, 4Bab, 6ab), having a refractive index lower than the refractive index of the inner cladding portion (4ab, 4Bab, 6ab), a part of the outer cladding portion (4ac, 6ac) of which in a longitudinal direction being removed; and at least one pumping light input-output optical fiber (4b, 6b) an end of which is fusion-spliced to the inner cladding portion (4ab, 4Bab, 6ab) of the main optical fiber (4a, 4Ba, 6a) at a portion where the outer cladding portion (4ac, 6ac) is removed, an average refractive index of which the pumping light input-output optical fiber in a contact region where the end of the pumping light input-output optical fiber (4b, 6b) is in contact with the inner cladding portion (4ab, 4Bab, 6ab) being larger than the refractive index of the inner cladding portion (4ab, 4Bab, 6ab), wherein the optical coupler is configured such that pumping light propagating in the pumping light input-output optical fiber (4b, 6b) is coupled to the side surface of the inner cladding portion (4ab, 4Bab, 6ab) from the contact region and propagates in the inner cladding portion (4ab, 4Bab, 6ab), or the pumping light propagating in the inner cladding portion (4ab, 4Bab, 6ab) is coupled from the side surface of the inner cladding portion to the contact region, and from the contact region to the pumping light input-output optical fiber (4b, 6b) and propagates in the pumping light input-output optical fiber (4b, 6b).