It is possible to achieve self-support of the fuel assembly without an upper grid plate when the fuel assembly is mounted or replaced, and it is also possible to prevent the fuel assembly from floating during a reactor operation. According to the present invention, the lower portion of the lower tie plate 7 as a part of the fuel assembly 3, which is inserted into the fuel support 9, extends, and a stable member 21 is provided around the extension portion 20, and thereby it is possible to achieve the self-support of the fuel assembly without the upper grid plate. In addition, since an increase in a weight due to extension of the lower portion of the lower tie plate 7 can prevent the floating during the reactor operation, a floating preventing mechanism using the upper grid plate is not necessary. Hence, it is possible to achieve the self-support of the fuel assembly without an upper grid plate when the fuel assembly is mounted or replaced, and it is also possible to prevent the fuel assembly from floating during the reactor operation.