A waveguide-type image rejection filter comprising: a waveguide-type 90-degree hybrid coupler (13, 13a, 13b) having an input port for receiving an input signal within a millimeter or submillimeter band, first and second output ports (P3, P4) for outputting divided signals, respectively, and a branch port for outputting a coupled signal composed of two signals inputted from the first and second output ports; a pair of band pass filters (15, 17): a waveguide-type first band pass filter whose one end is connected with the first output port and a waveguide-type second band pass filter whose one end is connected with the second output port; and a pair of radiowave absorptive terminations (19, 21): a first radio wave absorptive termination connected to the other end of the first band pass filter and a second radiowave absorptive termination connected to the other end of the second band pass filter, wherein the 90-degree hybrid coupler comprises a first waveguide; a second waveguide; a coupling section for coupling the first waveguide to the second waveguide, and a plurality of guiding branches arranged along the first and second waveguides, wherein the first waveguide has the input port at its end and the first band pass filter at the other end, and the second waveguide has the branch port at its end and the second band pass filter at the other end, wherein a shortest waveguide length between the coupling section and the first band pass filter along the first waveguide is the same as a shortest waveguide length between the coupling section and the second band pass filter along the second waveguide, wherein the first waveguide from the coupling section to the first band pass filter is configured to be a straight line and the second waveguide from the coupling section to the second band pass filter is configured to be a straight line, wherein the waveguide length meets the following formula: λ g 2 < d < λ g wherein d indicates a waveguide length, and λ g indicates a wavelength of an image band that should be intercepted in the waveguide: and wherein the 90-degree hybrid coupler and a pair of the band pass filters are configured as an integrated waveguide unit. a waveguide-type 90-degree hybrid coupler (13, 13a, 13b) having an input port for receiving an input signal within a millimeter or submillimeter band, first and second output ports (P3, P4) for outputting divided signals, respectively, and a branch port for outputting a coupled signal composed of two signals inputted from the first and second output ports; a pair of band pass filters (15, 17): a waveguide-type first band pass filter whose one end is connected with the first output port and a waveguide-type second band pass filter whose one end is connected with the second output port; and a pair of radiowave absorptive terminations (19, 21): a first radio wave absorptive termination connected to the other end of the first band pass filter and a second radiowave absorptive termination connected to the other end of the second band pass filter, wherein the 90-degree hybrid coupler comprises a first waveguide; a second waveguide; a coupling section for coupling the first waveguide to the second waveguide, and a plurality of guiding branches arranged along the first and second waveguides, wherein the first waveguide has the input port at its end and the first band pass filter at the other end, and the second waveguide has the branch port at its end and the second band pass filter at the other end, wherein a shortest waveguide length between the coupling section and the first band pass filter along the first waveguide is the same as a shortest waveguide length between the coupling section and the second band pass filter along the second waveguide, wherein the first waveguide from the coupling section to the first band pass filter is configured to be a straight line and the second waveguide from the coupling section to the second band pass filter is configured to be a straight line, wherein the waveguide length meets the following formula: λ g 2 < d < λ g wherein d indicates a waveguide length, and λ g indicates a wavelength of an image band that should be intercepted in the waveguide: and wherein the 90-degree hybrid coupler and a pair of the band pass filters are configured as an integrated waveguide unit.