at 35 db that is the socalled guard band u03b4f is equal to that in fig5 fig1 illustrates a specific operative example of the band rejection filter of 100 ghz in this example waveguides 42 and 43 both have an inner diameter r 1 of 28 mm and their lengths l 2 and l 3 are 115 mm and 135 mm respectively the distance d 1 between adjacent ones of coupling disks is about 18 mm and a half mirror 44 forming the hybrid 19 is made of fused quartz the resonator 45 comprises as coupling disks a polyester disk 47a an alumina disk 47b and a fused quartz disk 47c arranged in this order from the side of the hybrid 19 the resonator 46 comprises as coupling disks a fluoric resin disk 49a and a fused quartz disk 49b arranged in this order from the side of the hybrid 19 the frequency response of this filter produced as a trial is shown in fig1 in which the abscissa represents frequency and the ordinate loss the curve 57 indicates a measured value of the reflection characteristic and the curve 58 its theoretical value the curve 59 indicates a measured value of the pass characteristic and the curve 61 its theoretical value the measured values and the theoretical ones agree well with each other which indicates that the filter of this invention exhibits desired characteristics in this experiment the insertion loss is about 1 5 db but it can be expected theoretically to reduce the insertion loss to 0 2 db or so by using square te 10 over size waveguides as the waveguides 42.