The present invention relates to a photomask blank obtained by forming a resist film after performing a silylation process on a silicon-containing inorganic film and provides a method for manufacturing a photomask blank having at least a silicon-containing inorganic film over a transparent substrate and a resist film on the inorganic film, comprising: forming the silicon-containing inorganic film; heat treating the formed silicon-containing inorganic film at a temperature more than 200° C. under an atmosphere containing oxygen; performing a silylation process after the heat treatment; and then forming the resist film by application. The method can inhibit generation of defects due to resist residues or the like after development.