A multi-core fiber comprising: an even number of six or more of cores; and a clad surrounding outer circumferential surfaces of the cores, wherein: the even number of cores are formed of two types of cores between which an effective refractive index is different; and wherein in the cores, the two types of cores are alternately disposed at regular spacing, and the cores are annularly disposed at an interior angle formed of lines connecting centers of the cores adjacent to each other, the interior angle being greater than an angle of 90°; a difference in a mode field diameter of light propagating through the cores is 1 μm or less; characterized in that all cores are disposed on a single annulus; and an expression is satisfied: 1.450 × 10 − 5 × D ≤ Δn eff ≤ 0.002 wherein an inter-center pitch between the cores adjacent to each other is D μm and an effective refractive index difference between the cores adjacent to each other is Δn eff ; wherein 30 ≤ D ≤ 50 and Δn eff =n 1-1 - n 1-2 ; and wherein n 1-1 is a refractive index of the first type of core and n 1-2 is a refractive index of the second type of core . an even number of six or more of cores; and a clad surrounding outer circumferential surfaces of the cores, wherein: the even number of cores are formed of two types of cores between which an effective refractive index is different; and wherein in the cores, the two types of cores are alternately disposed at regular spacing, and the cores are annularly disposed at an interior angle formed of lines connecting centers of the cores adjacent to each other, the interior angle being greater than an angle of 90°; a difference in a mode field diameter of light propagating through the cores is 1 μm or less; characterized in that all cores are disposed on a single annulus; and an expression is satisfied: 1.450 × 10 − 5 × D ≤ Δn eff ≤ 0.002 wherein an inter-center pitch between the cores adjacent to each other is D μm and an effective refractive index difference between the cores adjacent to each other is Δn eff ; wherein 30 ≤ D ≤ 50 and Δn eff =n 1-1 - n 1-2 ; and wherein n 1-1 is a refractive index of the first type of core and n 1-2 is a refractive index of the second type of core . the even number of cores are formed of two types of cores between which an effective refractive index is different; and wherein in the cores, the two types of cores are alternately disposed at regular spacing, and the cores are annularly disposed at an interior angle formed of lines connecting centers of the cores adjacent to each other, the interior angle being greater than an angle of 90°; a difference in a mode field diameter of light propagating through the cores is 1 μm or less; characterized in that all cores are disposed on a single annulus; and an expression is satisfied: 1.450 × 10 − 5 × D ≤ Δn eff ≤ 0.002 wherein an inter-center pitch between the cores adjacent to each other is D μm and an effective refractive index difference between the cores adjacent to each other is Δn eff ; wherein 30 ≤ D ≤ 50 and Δn eff =n 1-1 - n 1-2 ; and wherein n 1-1 is a refractive index of the first type of core and n 1-2 is a refractive index of the second type of core . wherein an inter-center pitch between the cores adjacent to each other is D μm and an effective refractive index difference between the cores adjacent to each other is Δn eff ; wherein 30 ≤ D ≤ 50 and Δn eff =n 1-1 - n 1-2 ; and wherein n 1-1 is a refractive index of the first type of core and n 1-2 is a refractive index of the second type of core . <EOS>