Abstract: A diving bell type control rod of sodium bond type. The control rod including a plurality of control elements each comprising a pellet chamber for packing B.sub.4 C pellets and disposed in a cladding tube, an intermediate plug disposed above the pellet chamber, an upper chamber formed above the intermediate plug, a vent tube allowing the pellet chamber to communicate with the upper chamber while penetrating through the intermediate plug, upper and lower vent holes formed in upper and lower two stages in the cladding tube at the upper chamber, a sodium inflow port opening to the upper surface of the intermediate plug, and a sodium introduction tube extending from the sodium inflow port to a position below the lower end surface of the intermediate plug while penetrating through the intermediate plug.