In a magnetron sputtering apparatus an arrangement is made such that the peripheral portion of a target is uniformly eroded to attain a high efficiency in target utilization and, in addition, that an abnormal discharging hardly occurs to thereby enable satisfactory thin film forming. A magnet assembly is provided behind a target that is disposed opposite to the process substrate. This magnet assembly has a central magnet that is disposed linearly along the longitudinal direction, and a peripheral magnet that is disposed so as to enclose the periphery of the central magnet, while changing the polarity on the side of the target. At this time, among the respective magnetic fluxes generated between the central magnet and the peripheral magnet at the longitudinally end portions of the magnet assembly, the position at which the vertical component of the magnetic field becomes zero is locally shifted to the central magnet within a certain range.