A tungsten film forming method for forming a tungsten film on a surface of a target substrate by an ALD (atomic layer deposition) method comprises adding a reduction gas to allow an ALD reaction to mainly occur when a tungsten chloride gas is supplied. In the ALD method, the tungsten chloride gas as a tungsten source gas and the reduction gas for reducing the tungsten chloride gas are alternately supplied into a chamber which accommodates the target substrate and is maintained under a depressurized atmosphere, with a purge process for purging an inside of the chamber performed between the supply of the tungsten chloride gas and the supply of the reduction gas.