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A novel cavity-type multi-mode bandpass filter is developed. TEM-modes and TE-modes are degenerated in a single cavity. Up to quadruple-mode is possible in the principle. Because coupling between two TEM-modes is too strong to design an appropriate pass bandwidth, triple-mode resonator is analyzed for this time. By EM field simulation, the resonance modes are identified and three resonance frequencies are coincided at 2GHz by tuning the cavity structure. Further, adjustment methods for resonance frequencies, inter-resonator couplings, and input/output couplings are investigated to form a three-pole pseudo-Elliptic-function BPF. Then, an experimental triple-mode filter is fabricated. The measured performance shows very excellent performance with high selectivity owing to the attenuation poles at both sides of the pass-band. Thus, the validity of the novel multi-mode filter concept is confirmed.