PURPOSE: To simplify its manufacture and to attain light weight by providing at least one of spurious wave blocking metallic plates having at least an exciting element chip rows into a waveguide.

CONSTITUTION: When a fundamental wave S1 in a frequency f0 desired to be passed through is inputted to an input terminal of a band stop filter 11, a main electric field is vertical to the spuriour wave blocking metallic plate 12 and outputted from an output terminal 4B only with a slight effect. On the other hand, when a spurious wave S2 in a frequency 2f0 is inputted to the input terminal and the length of an exciting element chip 13 is odd-number times of λ/4 and the length of a coupling hole 15 is an integral number of times of λ/2, the electric field is affected largely, a part of the wave S2 is reflected in the power supply side and the rest is outputted to the end 4B. Thus, many rows of the element chips 13, 14 are arranged in the direction of the guide axis and the quantity of the wave S2 outputted to the end 4B is reduced to less value, then the filter acts as a blocking filter.