The invention provides a method of forming a magnetic layer with stable magnetic properties and stable recording-and-reproducing properties, by uniformizing the distribution of oxygen radical concentration upon reactive sputtering, and thereby uniformizing the concentration of oxygen to be taken into the magnetic layer along the plane direction. That is, the invention relates to a method of forming a magnetic layer by reactive sputtering, which comprises: placing a substrate in a reaction container; arranging a pair of electrode units comprising sputtering electrodes and targets which are disposed on surfaces of the sputtering electrodes and which contain chromium other than the oxide thereof, so that the electrode units respectively face both sides of the substrate while the targets are on the substrate sides; feeding an argon-water mixture gas to vicinities of the respective surfaces on the substrate sides of the pair of electrode units; and applying reactive sputtering so that the chromium other than the oxide thereof contained in the targets can be made into chromium oxide as a constituent to form the magnetic layer having the granular structure.