An ion beam assisted sputtering method for forming a film on the base material (2; 17) disposed in the film formation region by depositing the constituent particles of the target on the base material by using an ion beam assisted sputtering apparatus comprising: a target; a sputter ion source comprising a plurality of ion guns arranged so as to be able to irradiate the target from an end portion on one side to an end portion on the other side with sputter ion beams which sputter some of constituent particles of the target; a film formation region which has a rectangular shape and in which a base material for depositing thereon the particles sputtered from the target is disposed; and an assist ion beam irradiation device irradiating assist ion beams from a direction oblique to the direction of a normal of the film formation surface of the base material (2; 17) disposed in the film formation region, wherein: the target is formed in a rectangular shape so as to correspond to the film formation region; the plurality of ion guns are disposed along the longitudinal direction of the target and the film formation region; and the method comprising performing ion beam assisted sputtering by individually set current values of the individual ion guns such that the current value for generating the sputter ion beam of each of the ion guns disposed at both ends of the plurality of ion guns is set to be higher than the current value for generating the sputter ion beam of each of other ion guns disposed between said ion guns disposed at both ends, thereby irradiating the sputter ion beam from each of said other ion guns so that the ion beams overlap each other. a target; a sputter ion source comprising a plurality of ion guns arranged so as to be able to irradiate the target from an end portion on one side to an end portion on the other side with sputter ion beams which sputter some of constituent particles of the target; a film formation region which has a rectangular shape and in which a base material for depositing thereon the particles sputtered from the target is disposed; and an assist ion beam irradiation device irradiating assist ion beams from a direction oblique to the direction of a normal of the film formation surface of the base material (2; 17) disposed in the film formation region, wherein: the target is formed in a rectangular shape so as to correspond to the film formation region; the plurality of ion guns are disposed along the longitudinal direction of the target and the film formation region; and the method comprising performing ion beam assisted sputtering by individually set current values of the individual ion guns such that the current value for generating the sputter ion beam of each of the ion guns disposed at both ends of the plurality of ion guns is set to be higher than the current value for generating the sputter ion beam of each of other ion guns disposed between said ion guns disposed at both ends, thereby irradiating the sputter ion beam from each of said other ion guns so that the ion beams overlap each other. the target is formed in a rectangular shape so as to correspond to the film formation region; the plurality of ion guns are disposed along the longitudinal direction of the target and the film formation region; and the method comprising performing ion beam assisted sputtering by individually set current values of the individual ion guns such that the current value for generating the sputter ion beam of each of the ion guns disposed at both ends of the plurality of ion guns is set to be higher than the current value for generating the sputter ion beam of each of other ion guns disposed between said ion guns disposed at both ends, thereby irradiating the sputter ion beam from each of said other ion guns so that the ion beams overlap each other.