The present invention has for its object to provide a transgenic bird with a foreign gene containing a feline-derived protein-encoding sequence as transferred therein, and a method of producing the same. The present invention provides a method of producing a feline-derived protein by using a transgenic bird with a method which comprises infecting an avian embryo with a replication defective retrovirus vector containing a foreign gene by microinjection thereof into the early heart or blood vessel formed in the embryo and allowing the embryo to hatch.