A multicore optical fiber (10; 10A-10F) comprising: a core group consisting of eight or more cores; an inner cladding group consisting of inner claddings each of which individually surrounds a corresponding core out of the eight or more cores; a trench group consisting of trenches each of which individually surrounds a corresponding inner cladding out of the inner claddings; a common cladding individually surrounding each of the trenches; and a resin coating surrounding the common cladding, wherein each of the cores, each of the inner claddings, each of the trenches, and the common cladding are comprised of silica-based glass, wherein the diameter of the common cladding is not more than 126 μm, wherein where relative refractive index differences of each core, each inner cladding, each trench, the common cladding, and the coating with respect to a predetermined refractive index are defined as Δ1, Δ2, Δ3, Δ4, and Δ5, respectively, each core, each inner cladding, each trench, the common cladding, and the coating satisfy the following conditions: Δ5 > Δ1 > Δ2 > Δ3; and Δ1 > Δ4 > Δ3, wherein optical characteristics of each core are as follows: the transmission loss is not more than 0.5 dB/km at a wavelength of 1310 nm; the mode field diameter is from 8.0 μm to 10.1 μm at the wavelength of 1310 nm; a bending loss in a bending radius of not less than 5 mm or in the bending radius of not less than 3 mm and, less than 5 mm is not more than 0.25 dB/turn at the wavelength of 1310 nm; the zero dispersion wavelength is from 1300 nm to 1324 nm; the cable cutoff wavelength is not more than 1260 nm; and an inter-neighboring-core crosstalk between a core concerned and a core neighboring to the concerned core or a secondary inter-neighboring-core crosstalk between the concerned core and another core neighboring to the core neighboring to the concerned core is not more than 0.001 /km at the wavelength of 1310 nm, and the transmission loss of an outmost core having a core center located at a smallest distance from the coating, out of the eight or more cores, is larger than 0.5 dB/km at a wavelength of 1550 nm or at a wavelength of 1625 nm, or the transmission losses of the eight or more respective cores are larger than 0.4 dB/km at the wavelength of 1550 nm or at the wavelength of 1625 nm. a core group consisting of eight or more cores; an inner cladding group consisting of inner claddings each of which individually surrounds a corresponding core out of the eight or more cores; a trench group consisting of trenches each of which individually surrounds a corresponding inner cladding out of the inner claddings; a common cladding individually surrounding each of the trenches; and a resin coating surrounding the common cladding, wherein each of the cores, each of the inner claddings, each of the trenches, and the common cladding are comprised of silica-based glass, wherein the diameter of the common cladding is not more than 126 μm, wherein where relative refractive index differences of each core, each inner cladding, each trench, the common cladding, and the coating with respect to a predetermined refractive index are defined as Δ1, Δ2, Δ3, Δ4, and Δ5, respectively, each core, each inner cladding, each trench, the common cladding, and the coating satisfy the following conditions: Δ5 > Δ1 > Δ2 > Δ3; and Δ1 > Δ4 > Δ3, Δ5 > Δ1 > Δ2 > Δ3; and Δ1 > Δ4 > Δ3, wherein optical characteristics of each core are as follows: the transmission loss is not more than 0.5 dB/km at a wavelength of 1310 nm; the mode field diameter is from 8.0 μm to 10.1 μm at the wavelength of 1310 nm; the transmission loss is not more than 0.5 dB/km at a wavelength of 1310 nm; the mode field diameter is from 8.0 μm to 10.1 μm at the wavelength of 1310 nm; a bending loss in a bending radius of not less than 5 mm or in the bending radius of not less than 3 mm and, less than 5 mm is not more than 0.25 dB/turn at the wavelength of 1310 nm; the zero dispersion wavelength is from 1300 nm to 1324 nm; the cable cutoff wavelength is not more than 1260 nm; and an inter-neighboring-core crosstalk between a core concerned and a core neighboring to the concerned core or a secondary inter-neighboring-core crosstalk between the concerned core and another core neighboring to the core neighboring to the concerned core is not more than 0.001 /km at the wavelength of 1310 nm, and the transmission loss of an outmost core having a core center located at a smallest distance from the coating, out of the eight or more cores, is larger than 0.5 dB/km at a wavelength of 1550 nm or at a wavelength of 1625 nm, or the transmission losses of the eight or more respective cores are larger than 0.4 dB/km at the wavelength of 1550 nm or at the wavelength of 1625 nm.