This letter presents a design approach for a bandpass filter (BPF) using a novel triple-mode stripline resonator. We clarify the external Q factor with respect to the cutout size of the center conductor and feed-line position, to establish the design approach. It is found that external Q factors of each mode is handled respectively. From those results, a triple-mode BPF is designed and fabricated. The experimental results agree with the simulation results and accordingly it is confirmed that a design approach was established.