[en] An upper portion of a shaft extension mechanism is connected to a handling head joined with a control rod driving shaft, and the lower portion thereof is joined with an absorber portion. A heat sensitive cylinder constituting the shaft extension mechanism is disposed at the outer circumference of a control rod extension shaft. A less heat expansion rod connects the lower portion of the heat sensitive cylinder and a cam unit. The extension link mechanism is suspended from the handling head and absorber portion is disposed to the lower portion of the mechanism by way of a connection shaft. The pin of the extension link mechanism is engaged to a cam groove disposed to the cam unit. When the temperature of coolants in the reactor is elevated higher than usual, the cam unit moves downwardly by the difference of heat expansion between the heat sensitive cylinder and the low heat expansion rod to extend the extension link mechanism and urge the absorber portion downwardly. This can surely suppress a reactor core reactivity, thereby enabling to improve safety of a reactor system. (I.N.)