A fixing device comprising a fix member for touching an unfixed image on a recording medium, and a press member for being pressed against the fixing member, so as to fix the unfixed image on the recording medium by transporting the recording medium through a nip between the fix member and the press member, wherein (100K1).ltoreq.(K2).ltoreq.(320K1) where K1 is a heat transmission coefficient of the press member and K2 is a heat transmission coefficient of the fix member. With this arrangement, it is possible to transmit heat to toner and the recording medium in a short time. Thus, it is possible to attain high-speed printing.