Reflective mask blank, and method for manufacturing reflective mask

A reflective mask blank which is a material for a reflective mask used in EUV lithography using EUV light as exposure light, comprising a substrate, a multilayer reflection film that is formed on one main surface of the substrate and reflects the exposure light, a protection film formed in contact with the multilayer reflection film, and an absorber film that is formed on the protection film and absorbs the exposure light, wherein the multilayer reflection film has a periodically laminated structure in which low-refractive index layers composed of a material comprising molybdenum (Mo) and high-refractive index layers are alternately laminated, the low-refractive index layer consists of one or more of first low-refractive index sublayers, and one or more of second low-refractive index sublayers that have a different composition from a composition of the first low-refractive index sublayer, the first low-refractive index sublayer comprises molybdenum (Mo), and one or more elements selected from the group consis