PURPOSE: To simplify the observation by mounting an earth conductor on the back of a dielectric substrate while directional couplers of a microwave observing device at observing points of a microstrip line closer to each other to observe the transmission characteristic of a microwave as output of the coupler.

CONSTITUTION: A microwave observing device 20 is put tight on the first dielectric substrate 11 having a strip line 12 formed on the surface thereof while a 1/4 wavelength directional coupler 14 of the device 20 is arranged on the surface of the second dielectric substrate 13 having a earth conductor 18 mounted on the back thereof and terminals thereof are connected to a conductor 18 through a terminal resistance 15 and a through hole section 16. The device 20 is put tight on the substrate 11 with the coupler 14 and the line 12 closer to each other and a part of a microwave power propagating through the line 12 is drawn with the coupler 14 and led to an output terminal 103.