[en] An operation rod is disposed between a control rod main body and an extension tube for suspending thereof. A latch hook for engaging the control rod main body at the outer circumference is disposed at the lower top end of a heat sensitive member constituting the operation rod. A lower portion of bellows having the upper end closed and attached to the operation shaft is secured to the heat sensitive member at the inside of the heat sensitive member. The top end of the operation shaft is disposed at the inner side of the latch hook to prevent the latch hook from deforming inwardly and suspends the control rod main body. If temperature of coolants rises to elevate the temperature at the periphery of the heat sensitive material, liquid metal in the heat sensitive member expands to intrude into the outer circumference of the bellows. Therefore, the bellows are extended to move the operation shaft downwardly, the latch hook is deformed inwardly, and the control rod main body is dropped and inserted into the reactor core by the weight of its own. With such procedures, temperature elevation of coolants due to occurrence of an accident is rapidly detected, thereby enabling to rapidly insert the control rods to the reactor core. (I.N.)