To produce recombinant proteins and identify the gene functions, such as subcellular localization, one of the strongest agro-infiltration-based transient protein expression system for plant cells, termed ‘Tsukuba system’ has been established. By using this system, about 4 mg/g fresh weight of protein expression was detected in Nicotiana benthamiana. Furthermore, the system is also applicable to various plant species, including eggplant, tomato cultivars and wild species, lettuce, hot pepper, soybean, common bean, and orchid. The vector pBYR2HS contains a geminiviral replication system and a double terminator. Thus, the system is an effective and valuable tool for rapid transient expression to obtain a bunch of proteins and to examine gene function.