A fuel assembly has a regular dodecagon fuel rod arrangement with a single fuel rod provided to each apex of a regular dodecagon having lines of length A. Assuming a direction within a horizontal plane is transverse, and a direction perpendicular to the transverse direction is longitudinal, regular dodecagon fuel rod arrangements are arranged in regular intervals in the transverse direction and the longitudinal direction. In the transverse direction, two adjacent regular dodecagon fuel rod arrangements are arranged having opposing two sides of the regular dodecagons parallel to each other with the distance of mA, wherein m is a nonnegative integer, apart from each other. With respect to the longitudinal direction, two adjacent regular dodecagon fuel rod arrangements are arranged so that the opposing two sides of the regular dodecagons are parallel to each other with the distance of nA, wherein n is a nonnegative integer, apart from each other.