It is intended to provide a transgenic nonhuman mammal well representing the pathologic conditions of human rheumatoid arthritis. A transgenic nonhuman mammal is obtained by transferring a foreign DNA, wherein a DNA selected from the group consisting of an MHC class II transcriptional activator gene, the activity domain of an MHC class II transcriptional activator gene and a variant of an MHC class II transcriptional activator gene is provided under the control of a type II collagen promoter, into a cell at the early stage of development.