Disclosed is a novel means capable of producing a blood chimeric animal that maintains, for a long period of time, a state in which blood cells derived from a heterologous animal such as a human are retained at a high rate even in medium- to large-sized livestock. The method for preparing a non-human animal that retains blood cells derived from a heterologous animal, according to the present invention, includes transplanting, into the non-human animal, hematopoietic cells of the heterologous animal in which the function of a gene acting on a hematopoietic system has been modified. The gene acting on a hematopoietic system is, for example, the Lnk gene. According to the method of the present invention, when a medium- to large-sized mammal such as a pig is used as a recipient, the survival rate of hematopoietic cells derived from a heterologous animal is dramatically increased such that blood chimerism of 10% or more can be maintained even in a 16 month old animal.
