A covering material for solar thermal power generation system, characterized in that it is made of a film which has a tensile yield strength of at least 10 N/mm.sup.2 according to JIS K7127, a solar radiation transmittance of at least 85% according to JIS R3106, and a retention of at least 80% against the initial value of the tensile breakage strength after 5000 hours of the sunshine carbon arc lamp weather test according to JIS B7753.