PURPOSE: To contrive to heat indoors and at the same time economize in fuel by a constitution wherein a roof is made of hollow polycarbonate panel.

CONSTITUTION: A roof is made of hollow double layer polycarbonate panel 4a, in the hollow parts of which solar heat collecting rubber tubes 4b are arranged. The water sent by means of a pump 12 through a pipe 11 to a water leading-in header 7 passes through the tubes 4b and, while passing the tubes 4b, is heated by the solar heat. The heated water passes through a water leading-out header 5 and a pipe 9 to a heat accumulator 10. The indoor heating is performed by the hot water sent further from said heat accumulator 10 through a heat exchanger 13. By the way, because the titled hothouse is amply lighted during the day, the heat exchange through the heat exchanger may well be done only during the night.