Provided are a reflective mask blank and a reflective mask, which are able to reduce the shadowing effects of EUV lithography and form a fine pattern. As a result, a semiconductor device can be stably manufactured with high transfer accuracy. The reflective mask blank comprises a multilayer reflective film and an absorber film in that order on a substrate, and the absorber film comprises a material comprising an amorphous metal comprising at least one or more elements among cobalt (Co) and nickel (Ni).