PURPOSE: To improve the reliability of a microwave amplifier by impressing a voltage to a resistor pattern so as to burn out a contact part after a gold pattern is eliminated so as to eliminate a crack of an insulation board in adjusting an impedance of an input/output matching circuit.

CONSTITUTION: A ceramic cutter is used to shape a gold pattern at the boundary between an excess gold pattern part 51 and a gold pattern 5 to be left. Then a negative electrode terminal 12 is in contact with the gold pattern of the excess gold pattern part 51 and a positive electrode terminal 11 is in contact with the boundary after the gold pattern is removed aud a voltage is impressed between the positive and negative electrode terminals 12 and 11. Then the contact part of the positive electrode terminal 11 is dissolved and burnt out. Then the positive electrode terminal 11 is moved sequentially to burn out the contact part and all the excess gold pattern parts 51 are separated. Through the method above, the impedance is matched to eliminate the cracking of the insulation board 6 without cut flaw onto the insulation board 6.