It is an object of the present invention to provide a high affinity antibody effective as a diagnostic or therapeutic for various diseases; a transgenic mammal for producing the high affinity antibody; and a medicine comprising the high affinity antibody or a cell producing the high affinity antibody. According to the present invention, a transgenic mammal carrying a GANP gene transferred thereinto, its progeny, or a part thereof, and a method of producing a high affinity antibody using the same are provided.