Fusion partner cells that enable production of heterohybridomas even from cells of species other than mouse were produced by fusing myeloma cells derived from a first animal species with leukemia cells derived from a second animal species, which have an extra S phase in the cell cycle and have the property of diploidizing. Stable production of substances can be achieved by producing heterohybridomas through cell fusion between the fusion partner cells and substance-producing cells of an animal other than mouse.