The object is to provide a method for improving the engraftment ability of transplanted germ cells to the germ gland of a host and thus increasing the transplantation efficiency in transplanting the separated germ cells, in the case of, for example, farm-rising a surrogate parent, i.e., transplanting separated germ cells of a fish to a host fish and inducing the differentiation of the transplanted cells into a germ cell line. As a means for achieving the aforesaid object, disclosed is a method comprising intraperitoneally transplanting separated germ cells into a peri-hatching host fish individual, said separated germ cells being donated by a fish differing in family or species from the host fish, to thereby induce the differentiation of the separated germ cells into a germ cell line, wherein the transplanted host fish individual is raised in a temperature zone, which corresponds to the growth temperature range in the juvenile/fry stage of the fish donating the separated germ cells and in which the host fish can be raised, to thereby improve the engraftment ability of the separated germ cells to the germ gland of the host fish and increase the transplantation efficiency in transplanting the separated germ cells.
