The coat protein of lettuce big-vein virus (LBVV) was purified from highly purified LBVV, and its partial amino acid sequences were determined. An RNA encoding the coat protein of LBVV was cloned by polymerase chain reaction using primers designed based on the determined amino acid sequences information, and its primary structure was elucidated. Moreover, the present inventors succeeded not only in isolating RNA molecules of a plurality of LBVV-encoded proteins, including LBVV polymerase, by carrying out 3'RACE and 5'RACE using primers designed based on the resulting sequence information, but also in determining their primary structure. It was found that the use of these made it possible to produce lettuce resistant to LBVV and to diagnose infections with LBVV.