This paper studies a miniaturization of a substrate-integrated waveguide (SIW) bandpass filter (BPF) using only a square cavity. This cavity is to be loaded with a multi-sector circular patch, where each sectored patch is connected to the bottom surface of the cavity through a shorting blind via. Each of the shorting-via loaded sectored- patches and the cavity’s top and bottom surfaces form a resonator. Hence, multiple resonators can be housed in a single square cavity and then are fed properly to construct a multipole BPF. For easy integration with surrounding circuit components, itis to be considered by only the case where the cavity is fed with the coplanar waveguide (CPW) rather than the coaxial cable. The downshifts in the resonance frequency of the proposed resonator structure for the different number of sectors obtained from a complete cicuit patch are studied. BPFs constructed using one, two, and threesectored patches are designed and compared. A sample BPF using three sectored patches is fabricated and measured. As compared with the third-order BPF using three empty SIW cavities, the size reduction rate of the fabricated one is up to 98%. A good agreement is obtained between simulated results and those measured.