PURPOSE: To enable a T-type opening to be formed even without using an electron beam exposure device and mass production property to be increased by constituting a coated layer with a plurality of layers and increasing etching rate of layers out of a plurality of layers as they are further away from a semi-insulation substrate.

CONSTITUTION: A dummy gate 2 is formed on a semi-insulation substrate, for example a GaAs wafer 1. A first nitriding film 3 is formed in a state where the dummy gate 2 is coated and a second nitriding film 4 is formed on it. When chemical etching is performed, a second nitriding film 5, a first nitriding film 4, and a dummy gate 2 at a lower part of an opening 6 are eliminated. The first and second nitriding films 3 and 4 are formed so that the second nitriding film 4 has larger etching rate than the first nitriding film 3, the second nitriding film 4 with a larger etching rate can be etched more widely than the first nitriding film 3, thus enabling a T-type opening 7 which is wider at the upper part to be formed.