PURPOSE: To simply measure the conductivity of a conductive material, by inputting a microwave to a microwave-guide from one end thereof and inserting a specimen in the waveguide so as to cross the same to measure the transmissivity or reflectivity of the microwave.

CONSTITUTION: A microwave input means 7 is provided to the left end of a waveguide 1 and a slit (pref. as small as possible in its slit width) 2 crossing the waveguide 1 is provided on the way of the waveguide 1 and a sub-waveguide 2' is connected to the waveguide 1 on the left side of the slit 2 through a directional coupler 3 and detection means 5, 6 are provided to the terminal parts of the waveguides 1, 2'. When a conductive sheet containing a carbon fiber is inserted in the slit 2, the microwave is transmitted and reflected (absorbed) by the specimen and the output of the detection means 5 is lowered and output appears in the detection means 6. Since transmissivity and reflectivity are changed by the conductivity of the specimen, by separately performing calibration, conductivity can be performed. In the case of mere comparative measurement, the conductivity of the specimen can be compared by preparing a standard specimen.