PURPOSE: To eliminate the fluctuation of an attenuation characteristic caused by a temperature change by continuously providing thinner microstrip lines while branching them on a microstrip line, connecting and arranging a radio wave absorbing body on this branched microstrip.

CONSTITUTION: A microstrip line 3 is branched into microstrip lines 4 and 5 from a part (a), and the microstrip line 4 is opened at a part (b). The line width of the microstrip line 4 is enough thinner in comparison with that of the microstrip line 5, the microstrip lines 3 and 5 have the equal line width, and a radio wave absorbing body 1 is fixed in contact with the microstrip line 4. By transmitting one part of a transmitting signal to this branched microstrip line 4, this one part can be absorbed by the radio wave absorbing body 1 on this microstrip line 4. Thus, the change of the attenuation characteristic caused by the temperature change can be eliminated.