PURPOSE: To bore a small opening to an oxide film on a substrate, to realize short gate length and to obtain a J-FET having high mutual conductance and low input capacitance by forming the oxide film with a bird beak on a gate forming prearranged region and selectively boring the opening to the bird beak.

CONSTITUTION: When a laminated piece 13 formed through a pre-process is left on a gate forming prearranged region in an insular region 2A and a thermal oxide film 14 is formed on the surface through heating, a bird beak 14A by the thermal oxide film 14 is shaped under the laminated piece 13 while the insular region 2A is projected partially. The non-oxidizable film 13 is removed to expose the bird beak 14A, a resist film 15 is shaped on the thermal oxide film 14, and an opening 15a is formed on the bird beak 14A, and an opening 14a is formed to the bird beak 14A. The size of the opening 14a is controlled by the thickness of the thermal oxide film 14 at that time, and the position of the opening 14a is de- termined in a self-alignment manner by the position of the bird beak 14A.