be gradually narrowed from the shield to the substrate it is also possible to form the gap layer of a single material in such a way that the etching speed increases at the portion nearer the substrate in these methods the thickness of the gap wall is more easily controlled even if the gap width is narrower isotropic etching is used to form a gap wall by means of the shield a single etching may be applied to effect a vertical etching and an isotropic etching around and under the shield at a time however it is preferable that the gap layer is first vertically etched by using the shield as a mask and then the layer is isotropically etched around and under the shield to precisely control the pattern although various isotropic etching processes are known in the art reactive etching which is capable of controlling the etching condition to form the side surfaces of the gap wall vertical to the surface of the substrate is more advantageous according to the invention it is possible to easily form the gap width as short as less than 1 u03bcm or less than 0 5 u03bcm or less than 0 2 or even less than 0 1 u03bcm the magnetic pole layer may be formed not only of a single layer of a soft magnetic material but also of a multiple layer of a soft magnetic material laminated with a non magnetic insulating layer s therebetween the present invention will be described referring to the following examples a gap layer e g 1 u03bcm thick ti or sio 2 22 a shield.