A reflective mask blank comprises a multilayer reflective film for reflecting exposure light and formed on a substrate, a protective film for protecting the multilayer reflective film and formed above the multilayer reflective film, an absorber film for absorbing the exposure light and formed on the protective film, and a thermal diffusion-preventing film formed between the multilayer reflective film and the protective film. The protective film is made of ruthenium or a ruthenium compound containing ruthenium and at least one selected from the group consisting of molybdenum, niobium, zirconium, yttrium, boron, titanium, and lanthanum. The thermal diffusion-preventing film is made of a material having a refractive index of greater than 0.90 and an extinction coefficient of less than −0.020. A reflective mask comprises the reflective mask blank wherein the absorber film is formed with a pattern to be transferred to a transfer body.