PURPOSE: To efficiently execute curing with a small number of devices while preventing decrease of efficiency in paint spraying work.

CONSTITUTION: Guide rail support devices, each composed of a fixed support part 12, a support 14 and an arm 16, are removably attached to a parapet when paint spraying work is executed on flat roof slabs 2. Thereby a guide rail 30, composed in a straight line of guide rail units 18 supported with each of the guide rail support devices, is supported. A guide member 22 attached to the edge on the upper end of a curing sheet 20 is fitted to the guide rail 30, and the curing sheet 20 is moved along the guide rail 30 to a position nearest to a place on the flat roof slabs 2 where paint spraying is executed. Then the guide rail support devices on a position near a place where the paint spraying is finished are removed and attached again to a position near a place where the paint spraying is to be executed on the flat roof slabs 2, and the guide rail 30 is supported therewith together with the existing guide rail support devices.