PROBLEM TO BE SOLVED: To improve heat retaining properties and to obtain high-quality leaf lettuce in a high yield in a low-temperature period by forming a specific agricultural synthetic resin covering material so as to lay a coating film at the crop side, stretching the covering material and culturing leaf lettuce. SOLUTION: In culture of a horticultural crop using a greenhouse or a tunnel, an agricultural synthetic resin covering material having a transparent coating film made of a material having 1-30% blackbody radiation energy absorption ratio at 10μm thickness on one side of a synthetic resin base such as PE having 50-90% blackbody radiation energy absorption ratio at 100μm thickness is formed so as to lay the coating film at the side of a crop and stretched to carry out the culture of leaf lettuce. The thickness of the transparent coating film is 0.5-20μm and that of the agricultural synthetic resin covering material is preferably 50-500μm.
