PROBLEM TO BE SOLVED: To provide a method for keeping a plastic house warm by which the plastic house is kept warm by a light article capable of being extremely easily installed, with a reduced installation cost while saving the energy, and hardly causes dewfall when being kept warm, and further to provide a heat retention apparatus. SOLUTION: A lower part plastic sheet 2 is installed in the upper part of a plastic supporter 20, and one or more air mats 30 for heat retention are installed in the upper part of the lower plastic sheet 30 to form a heat-insulation facing part. In other aspect, an upper plastic sheet 4 is installed in the upper part of the air mat 30 for the heat retention to form a heat-insulation interlayer part between both of the upper and lower plastic sheets 2 and 4. Further, an intermediate plastic sheet is installed between the laminated and installed air mats 30 for the heat retention.
