PURPOSE: To make possible long operations with less wear resistance and loss and avoid the rise of dosage rate with Co 60 by forming a seal ring with alloy composed of C, N, Cr, Mn and Fe having specified composition.

CONSTITUTION: In the control rod driving mechanism for nuclear power generating plant in which a support drum 4 is installed on the inner side of a drum body 2 provided on the outside of an atomic reactor vessel 1 to support a control rod 3, pressed water is supplied from an inlet 7 or an inlet 10, the drum 4 is moved vertically, and control rod 3 is moved vertically, seal rings 5, 6 are provided to shut down the water for the control rod driving between an inside guide shaft 9 and an inner wall of the drum 4, and between a collet piston 11 and an external guide drum 15 respectively. The seal ring 5, 6 is composed of alloy including by weight, ≤0.3% C, 0.2W1.0% N, 10W30% Cr, 10W30% Mn and if necessary, further 0.5W3.0% V, the balance Fe and associated impurity, and having excellent wear resistance.