To provide a boiling/cooling device (heat diffusing block 1) which is hardly restricted by arrangement and excellent in mountability.

In the heat diffusing block 1, a hermetically sealed tank chamber 12 is formed by closing a hollow section formed in the block body 2 of the block 1 with two side plates. The tank chamber 12 is composed of a refrigerant chamber 12A which is largely extended in the transversal and longitudinal directions of the block body 2, and projecting heat radiating spaces 12B which are protruded to the upper part of the chamber 12A. A liquid refrigerant is poured in the chamber 12A to about the full height of the chamber 12A. In addition, a water passage section 15 is formed by closing the hollow section formed between the recessed and projecting sections and upper lid 4 of the block body 2 with two external plates 5 and cooling water flows through the section 15. The vapor of the refrigerant produced when the refrigerant boils and vaporizes upon receiving the heat of heating elements 13 is cooled by the cooling water flowing through the section 15 and condensed into droplets on the internal wall surface of the tank chamber 12 and the droplets return to the liquid refrigerant when the droplets drop into the refrigerant chamber 12A.