A Ge—Sb—Te film forming method includes a Sb source material introducing process, a first purging process, a Te source material introducing process, a second purging process, a Ge source material introducing process, a third purging process. An additive gas containing at least one of ammonia, methylamine, dimethylamine, hydrazine, monomethylhydrazine, dimethylhydrazine and pyridine is introduced in at least one of the Sb, Te and Ge source material introducing processes and the first to third purging processes.