The present invention aims at providing a circular waveguide polarizer with high performance and low cost. The circular waveguide polarizer is realized by arranging a plurality of side grooves 12 in a side wall of a circular waveguide 11 along the direction of a pipe axis C1 and by appropriately designing the number, spacing, radial depth, circumferential width, length in the pipe axis direction, and the like. According to this circular waveguide polarizer, disturbance is imparted to a section with a coarse electromagnetic field distribution in a transmission mode to create a phase delay, so that the amount of phase delay does not vary largely with a delicate change in width, depth and length of the side grooves 12. That is, there is little deterioration in characteristics caused by a machining error or the like, and hence it becomes possible to effect mass production and cost reductions.