Cooling device, electronic substrate and electronic device

A cooling device is provided. The cooling device is equipped with an electronic substrate on which a heating element has been mounted, and comprises a thermal diffusion unit of a plate-like shape. A front surface of the thermal diffusion unit thermally contacts a first circuit mounting surface of the electronic substrate. A rear face of the thermal diffusion unit thermally contacts a second circuit mounting surface of the electronic substrate. And, the thermal diffusion unit diffuses heat from the heating element according to vaporization and condensation principles of a refrigerant sealed therein.