Sunset at Chaophraya (Khu Kam) is a well-known novel about Japan written in Thai by the Thai author Thommayanti in 1965-1969. Set in Bangkok during World War II, the novel tells the tragic love story of Kobori, a Japanese soldier, and Angsumarin, a Thai woman. In this study, I analyze elements of Japonism in Sunset at Chaophraya by focusing on the composition of the‘ man from developed country—woman from the Orient’ dynamic to explore Thailand’s national image via the mirror theory. Four elements of Japonism are discussed: 1) The concept of the man from developed country improving the woman from the Orient. 2) The image of Japan, Musume, and Samurai, particularly the change in the Samurai’s image from savage to protector. 3) The motif of the intelligent Oriental woman trusted by the man from developed country. 4) The aspects of Kobori and Angsumarin’s location emphasize the Orient’s timelessness and exoticism, and that the man from developed country is assimilated into the life of the woman from the Orient. These elements, the dilution of Kobori’s ‘Japaneseness’, and Angsumarin’s pride as a Thai national, construct Japan and Thailand’s equal position, which indicates Thommayanti’s perception of the Thai nation.