The present invention has an object to provide an epitaxial film forming method of epitaxially growing a high-quality group III nitride semiconductor thin film on an α-Al2O3 substrate by a sputtering method. An epitaxial film forming method according to an embodiment of the present invention includes forming an epitaxial film of a group III nitride semiconductor thin film on an α-Al2O3 substrate placed on a substrate holder (111) including a heater electrode (104) and a bias electrode (103) in a sputtering apparatus (1) by applying high-frequency power to a target electrode (102) and applying high-frequency bias power to the bias electrode (103) while the heater electrode (104) maintains the α-Al2O3 substrate at a predetermined temperature. In this process, the high-frequency power and the high-frequency bias power are applied so that frequency interference therebetween may not occur.