PURPOSE: To keep the surface of snow in ski slopes under good-condition by a method in which a heat insulator is provided between the sides of expandible support parts made of a flexible bag and supplied with air to expand it ever the surface of snow.

CONSTITUTION: Expandible support parts 5 made of a flexible bag are set in parallel and heat insulators 10 of a flexible sheet are provided between the sides 9. A supply tube 2 is also provided along the direction of skiing on the end 32 of the widthwise direction crossing the sloping direction of snow surface 31 of the ski slope 30, and conduits 3 are provided with a given spacing L from the tube 2 to make up an air supplier. The support part 5 is rolled up, and its end is connected to the conduit 3 in such a way as to supply air through the tube 2 in order to expand it with the heat insulator 10 over the snow surface 31.