A multi-core optical fiber (300, 300A) for optical communications, comprising: plural cores extending along a predetermined axis direction; and a cladding surrounding each of the plural cores, wherein the cladding is comprised of silica glass doped with fluorine, and wherein each of the plural cores is comprised of silica glass doped with chlorine or pure silica glass, characterized by: leakage reduction portions each provided between an outer peripheral surface of the cladding and a position at which a distance from a center of the core is five-halves times a mode field diameter at a wavelength of 1.55 μm in a region including the core or between the outer peripheral surface of the cladding and a position at which an electric field amplitude in a periphery of the core is 10 -4 or less of the peak value thereof, and in that the cladding comprises an optical cladding provided on the outer periphery of the core and a physical cladding provided on the outer periphery of the optical cladding, wherein the leakage reduction portions being formed within the physical cladding, and wherein at least one of the leakage reduction portions is formed in the cladding so as to have a ring shape surrounding a core corresponding thereto among the plural cores on a cross-section perpendicular to the predetermined axis direction, and each of the leakage reduction portions has a first structure or a second structure, the first structure being defined by a refractive index profile such that a confinement factor of propagating light in a region surrounded by the leakage reduction portion is raised, the second structure being composed of a material that reduces power of propagating light, the material having a scattering coefficient greater than a scattering coefficient of the cladding. plural cores extending along a predetermined axis direction; and a cladding surrounding each of the plural cores, wherein the cladding is comprised of silica glass doped with fluorine, and wherein each of the plural cores is comprised of silica glass doped with chlorine or pure silica glass, leakage reduction portions each provided between an outer peripheral surface of the cladding and a position at which a distance from a center of the core is five-halves times a mode field diameter at a wavelength of 1.55 μm in a region including the core or between the outer peripheral surface of the cladding and a position at which an electric field amplitude in a periphery of the core is 10 -4 or less of the peak value thereof, and in that the cladding comprises an optical cladding provided on the outer periphery of the core and a physical cladding provided on the outer periphery of the optical cladding, wherein the leakage reduction portions being formed within the physical cladding, and wherein at least one of the leakage reduction portions is formed in the cladding so as to have a ring shape surrounding a core corresponding thereto among the plural cores on a cross-section perpendicular to the predetermined axis direction, and each of the leakage reduction portions has a first structure or a second structure, the first structure being defined by a refractive index profile such that a confinement factor of propagating light in a region surrounded by the leakage reduction portion is raised, the second structure being composed of a material that reduces power of propagating light, the material having a scattering coefficient greater than a scattering coefficient of the cladding.