A soft magnetic thin film of CoFe alloy having a high Br and low Hc is prepared by furnishing a plating tank including cathode and anode compartments which are separated by a diaphragm or salt bridge so as to permit charge transfer, but inhibit penetration of Fe ions, feeding a plating solution containing Co ions and divalent Fe ions to the cathode compartment, feeding an electrolyte solution to the anode compartment, immersing a substrate in the plating solution, immersing an anode in the electrolyte solution, electroplating, and heat treating the plated film at 100–550° C.; or by immersing a substrate and a soluble anode in a plating solution containing Co ions and divalent Fe ions, electroplating, and heat treating the plated film at 100–550° C.