PURPOSE: To continuously measure the moisture content of a continuously fed object to be inspected with high accuracy without stopping a machine by providing a microwave transmitter, microwave receiver, and transmissivity or reflectivity detecting means.

CONSTITUTION: Electromagnetic waves which are the microwave output of the antenna 3 of a microwave transmitter 2 are radiated upon an object 1 to be inspected from an opening 5 through a waveguide 4. The transmitted or reflected component of the electromagnetic waves is received by means of a reception antenna 9 through a waveguide 7 which guides the waves received by means of a microwave receiver 6. Since the moisture contained in the object 1 absorbs the energy of the microwave, the energy of the microwaves received by the receiver 6 changes depending upon the moisture content of the object 1. The intensity of the received microwaves relatively represents the moisture content of the object 1. A display 10 inputs the output of the antenna 9 and displays the intensity. In addition, when two receivers are installed in parallel at different positions and the outputs of both receivers are synthesized, the influence of fluctuation can be eliminated and the moisture content of the object 1 can be always measured with high accuracy in a stable state.