PROBLEM TO BE SOLVED: To provide a waterproof sheet fixing structure in which a fixing disc for fixing a waterproof sheet is fixed strongly to foundations without being deformed even if the fixing disc is fixed by a machine screw according to a heat insulation method since the fixing disc is disposed on a heat insulation material in a waterproof structure of a roof floor of a building that uses the heat insulation material and in such a case, there is a problem that the fixing disc may be deformed into concave shape if the fixing disc is fixed using the machine screw, and a construction method thereof.SOLUTION: A waterproof sheet fixing structure comprises a heat insulation material laid on foundations, a support post which is inserted from a surface of the heat insulation material toward the foundations and disposed on the foundations, a fixing disc which is disposed on the support post while being matched to an upper end portion of the support post, a fixing machine screw which is driven from a surface of t
