A solar collector comprising a heat collecting panel for receiving the sunlight, a cover glass arranged above the panel, and a sunlight-pervious diaphragm disposed between the panel and cover glass to form a closed space between itself and the cover glass, the diaphragm being adapted to expand and contract to vary the volume of the closed space, whereby the pressure in the closed space is kept substantially equal to the atmospheric pressure. With this arrangement, since the collector does not permit the entry of humid ambient air into the closed space, no condensates will be produced on the inner surface of the cover glass. Further, if the diaphragm is adapted to sag and contact the panel due to its expansion at the time of abnormal heating, heat dissipation from the panel is accelerated. Thus damage to the panel due to overheating can be prevented.