A fiber reinforced plastic (FRP) flat plate having no warps is continuously prepared by curing a continuous FRP plate molding material both top and back surfaces of which are covered with film-like sheetings by means of an ionizing radiation. In the process, the FRP molding material is continuously transported in lengthwise direction, and tensions are given to said film-like sheetings in both lengthwise and widthwise directions before and in the course of the irradiation.
