A combustion oscillation experiment showed combustion oscillation frequencies of around 350 Hz when only natural gas was used as fuel and approximately 200 and 400 Hz when a hydrogen–natural gas mixture was used. To analyze the resonant frequency, two- and four-region models considering unburned and burned regions of the combustor were developed. The experimental frequencies of the 100% natural gas condition were successfully predicted. Conversely, the experimentally observed frequencies under the hydrogen–natural gas condition were not accurately predicted. A swirler-combustor model was then constructed to get closer to the actual configuration and shape of the experimental setup. However, the model could not reproduce the experimental value under the hydrogen–natural gas condition. A whole piping model was then developed by adding a casing and an air supply pipe to the combustor. The resonant frequencies under both the 100% natural gas and hydrogen–natural gas conditions were successfully calculated. The model reproduced the range and change tendency of the experimentally measured oscillation frequency.