A multi-core optical fiber (10A, 10B, 10C) comprising: a plurality of core portions (100A, 100B) extending along a central axis, each of the core portions (100A, 100B) including a core (110A, 110B) extending along the central axis, an inner cladding (120A, 120B) surrounding an outer periphery of the core (110A, 110B), and a trench layer (130A, 130B) surrounding an outer peripheral surface of the inner cladding (120A, 120B); a common cladding surrounding an outer peripheral surface of the trench layer (130A, 130B) of each of the plurality of core portions (100A, 100B) and having an outer diameter of 124 μm or more and 181 μm or less; wherein the multi-core optical fiber (10A, 10B, 10C) includes a linear array group (G0, G1, G2, G3, G4) constituted by at least four core portions (100A, 100B), each of the at least four core portions (100A, 100B) having a core center (AX1, AX2) being located on a straight line (L0, L1, L2, L3, L4) defined on a cross section of the multi-core optical fiber (10A, 10B, 10C), the cross section being orthogonal to the central axis, and the at least four core portions (100A, 100B) include a first core portion and a second core portion adjacent to each other and having a spacing A [μm] between individual core centers (AX1, AX2), relative refractive index differences of the first core portion and the second core portion with respect to a refractive index of the common cladding differ by 0.001Λ [%] or more, each of the at least four core portions (100A, 100B) constituting the linear array group (G0, G1, G2, G3, G4) has a refractive index profile in which at least a relative refractive index difference between the core (110A, 110B) and the inner cladding (120A, 120B) is equal to the others; the multi-core optical fiber (10A, 10b, 10C) further comprises a resin coating having an outer diameter of 195 μm or more and 250 μm or less while surrounding an outer peripheral surface of the common cladding. a plurality of core portions (100A, 100B) extending along a central axis, each of the core portions (100A, 100B) including a core (110A, 110B) extending along the central axis, an inner cladding (120A, 120B) surrounding an outer periphery of the core (110A, 110B), and a trench layer (130A, 130B) surrounding an outer peripheral surface of the inner cladding (120A, 120B); a common cladding surrounding an outer peripheral surface of the trench layer (130A, 130B) of each of the plurality of core portions (100A, 100B) and having an outer diameter of 124 μm or more and 181 μm or less; wherein the multi-core optical fiber (10A, 10B, 10C) includes a linear array group (G0, G1, G2, G3, G4) constituted by at least four core portions (100A, 100B), each of the at least four core portions (100A, 100B) having a core center (AX1, AX2) being located on a straight line (L0, L1, L2, L3, L4) defined on a cross section of the multi-core optical fiber (10A, 10B, 10C), the cross section being orthogonal to the central axis, and the at least four core portions (100A, 100B) include a first core portion and a second core portion adjacent to each other and having a spacing A [μm] between individual core centers (AX1, AX2), relative refractive index differences of the first core portion and the second core portion with respect to a refractive index of the common cladding differ by 0.001Λ [%] or more, each of the at least four core portions (100A, 100B) constituting the linear array group (G0, G1, G2, G3, G4) has a refractive index profile in which at least a relative refractive index difference between the core (110A, 110B) and the inner cladding (120A, 120B) is equal to the others; the multi-core optical fiber (10A, 10b, 10C) further comprises a resin coating having an outer diameter of 195 μm or more and 250 μm or less while surrounding an outer peripheral surface of the common cladding.