A fuel assembly comprises a plurality of fuel rods which contain nuclear fuel material inside, a lower tie plate which holds the lower end of the fuel rods and has a path inside to lead coolant between the fuel rods, and a channel box which encloses a bundle of the fuel rods. An orifice, in which a plurality of round rods are arranged to cross the coolant flow path, is installed in a through hole at a side wall of the lower tie plate by connecting to the side wall. Orifice coefficient of the orifice becomes large at small flow rate of coolant which supplied to the fuel assembly, and becomes small at large flow rate of coolant. By using the fuel assembly described above, void fraction in a gap region between fuel assemblies can be altered during beginning and end of an operation cycle of the nuclear reactor.