An ultra-slow muon beam, which has been developed in KEK-MSL and RIKEN/RAL since 1985, will be utilized for promoting the various advanced researches. Positive muons are generated in the primary proton beam line, and introduced into a tungsten foil which is heated at 2000K Positive muons will capture electrons and will be converted into muoniums. Then the thermal muoniums are evaporated into an ultra-high vacuum and ionized by a laser ionization method. A highly pure tungsten foil is essential, because the clean surface of tungsten is required to obtain the intense muoniums. Unfortunately, the companies, which had supplied the highly pure (6 N-99.9999 %) tungsten foil, discontinued the manufacturing. It used to be manufactured by a hot rolling technique of the highly pure tungsten bulk material. Therefore the development of manufacturing methods was started instead of fabricating the new and expensive hot rolling apparatus. To know the feasibility of the manufacturing, several decades of 0.2-mm thick plates made of ordinarily pure tungsten material (3 N-99.95%) was utilized. Then, the thickness is reduced up to 50 µm by two methods, the etching and the mechanical polishing respectively. As the results, uniform 50-µm thick foils could be successfully obtained by the mechanical polishing. As the next step, the successful mechanical polishing was introduced to the highly pure tungsten bulk. In this article, the development of the manufacturing method of the highly pure tungsten foil for thermal muonium generation will be described.