A method for forming a thin film of the present invention includes the steps of performing sequentially first idling discharge under application of a first level of discharge power upon gas introduction (period t1a), intermediate idling discharge under application of a second level of discharge power lower than the first level (period t1b), and second idling discharge at the first level of discharge power (period t1c), so that the pressure of sputtering gas and an intermediate product produced during the idling discharge are stabilized. The discharge power is lowered to the second level, the shutter is opened, and an ITO thin film, for example, is formed on a first substrate (t2). Thereafter, the same cycles are repeated to form ITO thin films on substrates.