PURPOSE: To obtain satisfactory characteristics by arbitrarily changing the width of the strip line in the state of fixing a specified impedance by changing the width of a ground conductor corresponding to the width of a transmission line conductor.

CONSTITUTION: A transmission line conductor 101 and a ground conductor 102 are respectively grounded to both flanks of an alumina substrate 100 being a dielectric substrate. The center lines of both conductors 101 and 102 in a lateral direction are coincident concerning the direction. Then, width w1 of the transmission line 101 may be changed into any arbitrary value and in this case, the width w2 of the ground conductor 102 may be changed so as to satisfy the conduction of turning the specified impedance to be obtained to a specified value corresponding to the change of the width w1 of the transmission line conductor 101 by suitably setting the thickness of the alumina substrate 100. Therefore, the width w1 of the transmission line conductor 101 can be changed into the desired value while keeping the specified impedance constant and microwave dispersion and reflection caused by connection with any other device are decreased.