It is provided a simple and efficient process for producing a large amount of protein in a cell-free protein synthesis system, through a continuous synthesis reaction over a long time using a linear DNA as a template. The continuous synthesis reaction is carried out for at least one hour with a cell-free protein synthesis system comprising an extract of Escherichia coli cells cultured at any temperature between 18° C. and 36° C., and a linear template DNA encoding the protein. The continuous protein synthesis system is preferably a dialysis system or a continuous flow system. The cell-free protein synthesis system by dialysis system comprises an internal dialysate comprising the extract and the linear template DNA, an external dialysate comprising a substrate solution for protein synthesis, and a dialysis membrane through which the substrate can be transferred.