In an initial core of the present invention, a low enrichment fuel assembly having the lowest average enrichment factor and three fuel assemblies having a higher average enrichment factor than that of the low enrichment fuel assembly are arranged in a square shape, control rods of a cross shape are arranged at each of four corners of the square shape to constitute a unit loading pattern, and a plurality of the unit loading patterns are provided in the central region of the core, the fuel assemblies having the higher average enrichment factor are divided by a diagonal line into a first region of the side of the control rods and a second region of the side opposite to the control rods, and the number of the gadolinia-containing fuel rods is greater in the second region by at least two than in the first region. The initial core having an enhanced average enrichment factor for accomplishing a high exposure, makes it possible to maintain thermal margin, to suitably suppress the excess reactivity and to improve fuel economy.