Disclosed is a technique for increasing the quantity of a plant biomass to a great extent. The technique involves introducing both a gene which encodes protein phosphatase 2C that comprises three consensus sequences respectively comprising the amino acid sequences depicted in SEQ ID NO:1 to SEQ ID NO:3 lying from the N-terminal in this order and a gene which encodes glutathione-bound plastid-type fructose-1,6-bisphosphate aldolase, or modifying an endogenous expression regulation region for the gene.