A thin-film manufacturing method includes the steps of: generating a plasma from source gas; extracting ions from the plasma; and depositing a thin film on one side or both sides of a substrate to be deposited with the ions. The method is performed in an apparatus including: a plasma chamber generating the plasma; a film deposition chamber accommodating the substrate to be deposited; an ion transfer path for transferring the ions from the plasma chamber to the film deposition chamber; a branch pipe branching from the ion transfer path; and an exhaust system connected to the branch pipe. The thin film is formed while the source gas except the ions is exhausted from the branch pipe.