PURPOSE: To radiate the heat produced by a functional element more efficiently in a stable state by providing radiation fins to the outer surface of the cavity section of a sealed container.

CONSTITUTION: The heat produced by a semiconductor element 2 is absorbed by the working liquid 4 in a liquid reservoir 3a adjacent to the element 2 and the vapor of the liquid 4 produced by the heat of the element 2 moves upward and reaches a cavity section 3b. After reaching the cavity 3b, the vapor further moves toward the end of the cavity 3b along the upper surface of the cavity 3b. While the vapor move in such way, the heat of the vapor is removed by radiation fins 5 and the vapor is condensed to the liquid which returns to the liquid reservoir 3a along the lower surface of the cavity 36. As the internal heat movement by the vaporization and liquefaction of the working liquid 4 continues, the semiconductor element 2 which is the heat source is effectively cooled. Therefore, the heat of a functional element can be radiated to the outside effectively and stably and the reliability, etc., of the element can be improved.