PURPOSE: To improve the transmission efficiency of a microwave current by increasing the surface area of a microstrip line to reduce the apparent surface resistance.

CONSTITUTION: A dielectric substrate 1 contains a flat ground layer 2 on one of its both sides and the projecting dielectric members 4 extending in the current flowing direction on the other side respectively. Then a strip line 3 consisting of a dielectric member is laminated on an area including the members 4. So that an uneven outer circumference is secured for a section which is vertical to the current flowing direction of the line 3.