Dual-comb spectroscopy using two frequency combs with slightly different repetition frequencies has been attracting attention as a new spectroscopic tool. Dual-comb spectroscopy can be considered as a Fourier transform spectroscopy without the mechanical delay-stage. Dual-comb spectroscopy has much higher data acquisition rate and frequency resolution, compared with conventional Michelson interferometerbased Fourier transform spectroscopy. In this paper, we introduce the principle of dual-comb spectroscopy and novel way for phase locking between two frequency combs for simple dualcomb system. In addition, the application of dual-comb spectroscopy to FBG strain sensor is described.