The present invention relates to a small cobra antenna that has a high performance as an antenna gain, and minimizes the effect of the length of the coaxial wire. The first voltage supply circuit includes a resistor and a capacitor, the resistance and capacitance of which are determined so that a first voltage reaches a reference voltage within a reference time. An end of the field insulating film has a convex shape in a cross section perpendicular to the main surface of the substrate, and an upper surface of the field insulating film is rougher than an upper surface of a portion of the gate wire below which the field insulating film is not disposed.