An apparatus for producing a mass of water vapor includes a water film maker and a device which prevents the water film maker from drifting. The water film maker floats on a surface of a water source and absorbs water of the water source by virtue of a capillary mechanism or a moisture-holding ability to make a thin water film on a surface of the water film maker. The thin water film is exposed to sunlight. The water film maker comprises a first hollow body capable of being filled with air, and a sheet having said capillary mechanism or having said moisture-holding ability, said sheet covering said first hollow body. Water in the thin water film is vaporized by sunlight energy to thereby produce a mass of water vapor in the sky.