This invention is intended to increase biomass production through suppression of expression of a gene functioning in relation to intracellular vesicular transport or inhibition of functions of a protein encoded by such gene. To this end, expression of a gene encoding the coatomer adapter zeta subunit is suppressed, the coatomer adapter zeta subunit is inhibited, expression of a gene encoding the clathrin adaptor small (sigma) subunit is suppressed, or the clathrin adaptor small (sigma) subunit is inhibited.