This report describes an experimental method to find a boundary between a phase-locking state and a suppression state in the Arnold tongue of 1: 1 forced synchronization. A thermoacoustic oscillator in a gas-filled resonance tube is used as a test system. It generates longitudinal acoustic oscillations by imposing an axial temperature difference on the tube. While pressure oscillations synchronous to the external periodic force are observed commonly in these regions, distinctive behaviors are found in the transient phase portraits after the synchronized state is temporarily perturbed. Based on the experimentally obtained results, a saddle-node bifurcation line was discovered in the Arnold tongue of the forced thermoacoustic oscillator. The resulting bifurcation diagram is compared with that of the theoretical model of the thermoacoustic oscillator.