A multi-core fiber (1) is a multi-core fiber including 10 or greater of even numbered cores and a cladding surrounding the core. In the even numbered cores, a half of cores (11a) are disposed in such a manner that centers are located on the apexes of a regular polygon (RP) whose center is at an origin point (O) in a cladding (20). In the even numbered cores, other cores (11b) are disposed in a manner that centers are located on perpendicular bisectors (LV) of the edges of a regular polygon on the inner side of the regular polygon (RP). The other cores (11b) are disposed in a specific range in the regular polygon (RP).