A multi-core optical fiber (50, 51) comprising: a ring-shaped common trench having a refractive index lower than a refractive index of a cladding and surrounding all cores in a cross section, wherein a minimum value E, in μm, of the distance from a boundary between each core and the cladding to an inner diameter of the common trench satisfies Formula C1, wherein the cut-off wavelength of each core is shorter than 1260 nm, characterised in that: the common trench has an inner diameter C, in μm, and a ring width W, in μm, that satisfy a trench volume x of Formula C2, and a desired bending loss α B0 of each core at a bending radius of 30 mm and a wavelength of 1625 nm is lower than 0.1 dB/100 turns: [Formula C1] E > − 0.35542 − 35.576 Δ _ − 93.643 Δ_ 2 − 86.407 Δ_ 3 where Δ_ is a relative index difference, in %, of the common trench with respect to the cladding: [Formula C2] α B × 10 − Δ α B 10 < α B 0 λ cc + Δλ < λ cc 0 Δ α B = 0.043771 x − 1.3385 × 10 − 5 x 2 + b When x < 1250 μm 2 % Δ α B = 18.114 + 0.012932 x + b When x ≥ 1250 μm 2 % Δ λ = 0.10453 x + 0.00010793 x 2 b = 45.3 − 0.56 α B where x, in μm 2 %, is a product of an area of the common trench in the cross section of the multi-core optical fiber and an absolute value Δ_ of the relative index difference of the common trench, α B0 is a desired bending loss, in dB/100 turns, of each core, λ cc0 is a desired cutoff wavelength, in nm, of each core, α B is a bending loss, in dB/100 turns, of each core in a case where there is no common trench, and λ cc is a cutoff wavelength, in nm, of each core in a case where there is no common trench. a ring-shaped common trench having a refractive index lower than a refractive index of a cladding and surrounding all cores in a cross section, wherein a minimum value E, in μm, of the distance from a boundary between each core and the cladding to an inner diameter of the common trench satisfies Formula C1, wherein the cut-off wavelength of each core is shorter than 1260 nm, characterised in that: the common trench has an inner diameter C, in μm, and a ring width W, in μm, that satisfy a trench volume x of Formula C2, and a desired bending loss α B0 of each core at a bending radius of 30 mm and a wavelength of 1625 nm is lower than 0.1 dB/100 turns: [Formula C1] E > − 0.35542 − 35.576 Δ _ − 93.643 Δ_ 2 − 86.407 Δ_ 3 where Δ_ is a relative index difference, in %, of the common trench with respect to the cladding: [Formula C2] α B × 10 − Δ α B 10 < α B 0 λ cc + Δλ < λ cc 0 Δ α B = 0.043771 x − 1.3385 × 10 − 5 x 2 + b When x < 1250 μm 2 % Δ α B = 18.114 + 0.012932 x + b When x ≥ 1250 μm 2 % Δ λ = 0.10453 x + 0.00010793 x 2 b = 45.3 − 0.56 α B where x, in μm 2 %, is a product of an area of the common trench in the cross section of the multi-core optical fiber and an absolute value Δ_ of the relative index difference of the common trench, α B0 is a desired bending loss, in dB/100 turns, of each core, λ cc0 is a desired cutoff wavelength, in nm, of each core, α B is a bending loss, in dB/100 turns, of each core in a case where there is no common trench, and λ cc is a cutoff wavelength, in nm, of each core in a case where there is no common trench. the common trench has an inner diameter C, in μm, and a ring width W, in μm, that satisfy a trench volume x of Formula C2, and a desired bending loss α B0 of each core at a bending radius of 30 mm and a wavelength of 1625 nm is lower than 0.1 dB/100 turns: [Formula C1] E > − 0.35542 − 35.576 Δ _ − 93.643 Δ_ 2 − 86.407 Δ_ 3 where Δ_ is a relative index difference, in %, of the common trench with respect to the cladding: [Formula C2] α B × 10 − Δ α B 10 < α B 0 λ cc + Δλ < λ cc 0 Δ α B = 0.043771 x − 1.3385 × 10 − 5 x 2 + b When x < 1250 μm 2 % Δ α B = 18.114 + 0.012932 x + b When x ≥ 1250 μm 2 % Δ λ = 0.10453 x + 0.00010793 x 2 b = 45.3 − 0.56 α B where x, in μm 2 %, is a product of an area of the common trench in the cross section of the multi-core optical fiber and an absolute value Δ_ of the relative index difference of the common trench, α B0 is a desired bending loss, in dB/100 turns, of each core, λ cc0 is a desired cutoff wavelength, in nm, of each core, α B is a bending loss, in dB/100 turns, of each core in a case where there is no common trench, and λ cc is a cutoff wavelength, in nm, of each core in a case where there is no common trench.