An object of the invention is to effectively prepare a fish embryo with a correct chromosomal ploidy by nuclear transplantation in which an exogenous fish nucleus is transplanted in a cytoplasmic recipient. For this object, the invention comprises a step of preparing a fish embryo by transplanting a fish cell nucleus to an unfertilized egg. The step of preparing a fish embryo comprises a step of imposing physical and/or chemical stress to the unfertilized egg after activation. By imposing such stress, the stage of haplosis in a female nucleus which happens at the early stage of a series of developmental steps occurring in an unfertilized egg is suppressed and the correct ploidy of an obtained embryo is at least secured.