A multicore optical fiber with four cores having a radius a 1 disposed in a longitudinal direction in square lattice shape, comprising: first cladding regions disposed around each of the cores and having a radius a 2 with a refractive index lower than a refractive index of the cores, a relative refractive index difference to the cores being Δ 1; and a second cladding region disposed on an outer periphery of the first cladding regions and having a refractive index higher than the refractive index of the first cladding regions and lower than the refractive index of the cores, a relative refractive index difference to the core being Δ 2, wherein an outer diameter of the second cladding region is 125±1 μm, bending loss at a wavelength of 1625 nm and a bending radius of 30 mm is 0.1 dB/100 turns or less, and a cutoff wavelength is 1530 nm or less, wherein the multicore optical fiber comprises a region having a width w and having a same refractive index as the refractive index of the second cladding region between each of the cores and the first cladding region, characterized in that the width w is 0 μm<w≤1.3 μm; wherein: a 2 /a 1 = 2.0 and an effective cross-sectional area at a wavelength of 1550 nm is 80 μm 2, and when the relative refractive index difference of the first cladding regions is represented by an axis of abscissas and the relative refractive index difference of the second cladding region is represented by an axis of ordinates, Δ 1 is a X coordinate and Δ 2 is a Y coordinate, wherein (X, Y) = (Δ 1, Δ 2) is within a range enclosed by: A0 (-0.800, -0.396) A1a(-0.800, -0.340) A1b(-0.750, -0.351) A1 (-0.682, -0.378). first cladding regions disposed around each of the cores and having a radius a 2 with a refractive index lower than a refractive index of the cores, a relative refractive index difference to the cores being Δ 1; and a second cladding region disposed on an outer periphery of the first cladding regions and having a refractive index higher than the refractive index of the first cladding regions and lower than the refractive index of the cores, a relative refractive index difference to the core being Δ 2, wherein an outer diameter of the second cladding region is 125±1 μm, bending loss at a wavelength of 1625 nm and a bending radius of 30 mm is 0.1 dB/100 turns or less, and a cutoff wavelength is 1530 nm or less, wherein the multicore optical fiber comprises a region having a width w and having a same refractive index as the refractive index of the second cladding region between each of the cores and the first cladding region, characterized in that the width w is 0 μm<w≤1.3 μm; wherein: a 2 /a 1 = 2.0 and an effective cross-sectional area at a wavelength of 1550 nm is 80 μm 2, and when the relative refractive index difference of the first cladding regions is represented by an axis of abscissas and the relative refractive index difference of the second cladding region is represented by an axis of ordinates, Δ 1 is a X coordinate and Δ 2 is a Y coordinate, wherein (X, Y) = (Δ 1, Δ 2) is within a range enclosed by: A0 (-0.800, -0.396) A1a(-0.800, -0.340) A1b(-0.750, -0.351) A1 (-0.682, -0.378). a 2 /a 1 = 2.0 and an effective cross-sectional area at a wavelength of 1550 nm is 80 μm 2, and when the relative refractive index difference of the first cladding regions is represented by an axis of abscissas and the relative refractive index difference of the second cladding region is represented by an axis of ordinates, Δ 1 is a X coordinate and Δ 2 is a Y coordinate, wherein (X, Y) = (Δ 1, Δ 2) is within a range enclosed by: A0 (-0.800, -0.396) A1a(-0.800, -0.340) A1b(-0.750, -0.351) A1 (-0.682, -0.378). A0 (-0.800, -0.396) A1a(-0.800, -0.340) A1b(-0.750, -0.351) A1 (-0.682, -0.378).