In this application, the provided are: a Down syndrome rat model characterized in that a rat gene homologous to at least one gene present on a human chromosome 21 or fragment thereof is a trisomy and is transmittable to progeny; or a Down syndrome rat model characterized in that it comprises a human chromosome 21 or fragment thereof, or an exogenous rat chromosome or fragment thereof on which a rat gene homologous to the human chromosome 21 or fragment thereof is present, wherein at least one gene on the human chromosome 21 or fragment thereof or on the exogenous rat chromosome or fragment thereof is added to endogenous rat genes homologous to the at least gene so as to become a trisomy and to be transmittable to progeny: and a method for producing the Down syndrome rat model.