An image suppression mixer of the waveguide type is disclosed which includes a waveguide signal input terminal at one end of a waveguide, a local oscillation input terminal at the other end of the waveguide, a mixer diode inserted within the waveguide and an IF signal output terminal connected to the mixer diode. At least the width of the waveguide on the side of the waveguide signal input terminal is selected to be a cutoff dimension for the image frequencies f.sub.m and the local oscillation frequency f.sub.1 the waveguide signal frequencies f.sub.2, the local oscillation frequency f.sub.1 and the image frequencies f.sub.m are correlated as f.sub.m