To provide a microwave band attenuator which can facilitate the control of attenuation by providing a radio wave absorbent on a microstrip line formed on a dielectric substrate and controlling the size of the absorbent.

A conductor 9 is formed on the back side of a dielectric substrate 7, and a microstrip line 8 is formed on the surface side of the substrate 7 respectively. Then a radio wave absorbent 10 is provided on the line 8 formed on the surface of the substrate 7. When a microwave flows to the line 8, an electromagnetic field is mostly concentrated on the substrate 7 and also partly distributed above the substrate 7. As the absorbent 10 is prepared at the part where the electromagnetic field is concentrated or distributed, the energy passes the said part is absorbed by the absorbent 10 and attenuated. Then the attenuation is controlled through control of the size of the absorbent 10.