PURPOSE: To provide the parallel polarization polarizer almost completely preventing the leakage of the received radio waves to the transmission terminal side and easy to take the impedance matching of a coupling hole.

CONSTITUTION: A coupling hole 4, having a dipole antenna 4a of 1/4-length of the wave length of a low frequency band, resonates at a low frequency band and prevents the leakage of the received radio wave to a low-pass preventing filter 3 almost completely. A high-pass preventing filter 2 is of a corrugated construction whose corrugated depth is nearly 1/4-length of the guide wave length (λg) in the high-frequency band and whose corrugated width is the length obtaining the required filter characteristic. An equivalent short surface is in the nearest corrugated groove 2a. The coupling hole will not be affected by electrical influence from the corrugated groove 2a in the high-band preventing filter in the case of heading for a primary radiator 11 from this equivalent short surface and located at the λg/4. Thus, the impedance matching can be taken easily.