A multi-core optical fiber which has a plurality of core portions arranged separately from one another in a cross-section perpendicular to a longitudinal direction, and a cladding portion located around the core portions, the multi-core optical fiber comprises a cylindrical portion of which diameter is even, and a reverse-tapered portion gradually expanding toward at least one edge in the longitudinal direction, wherein a gap between each adjacent ones of the core portions in the reverse-tapered portion is greater than that in the cylindrical portion.