Abstract: A gene encoding a production amount-potentiating factor is introduced into an animal cell to transform the cell. Alternatively, a protein production gene and the gene encoding the production amount-potentiating factor are introduced into the animal cell to transform the cell. Herein, as the production amount potentiating factor, there is used a factor having caspase activity inhibiting activity and/or protein biosynthesis activity potentiating action, for example, baculovirus P35. Further, the animal cell is cultured by a culturing method under a condition that apoptosis is not induced, so that a protein is mass-produced.