When two periodic disturbing forces with frequencies Ω1 and Ω2 (Ω2>Ω1) respectively are applied to a vibratory system with the unsymmetrical spring force, the combination tones of the differential type having frequency Ω=Ω2-Ω1 occur. In the present paper, the combination tones of differential type are discussed analytically and experimentally, and it is found that there are three types of the amplitude-frequency response curve of the combination tones of differential type as well as of summed type, but the relations between these types of the response curves and the ratios of the forcing frequencies K=Ω2/Ω1 are different from those of the combination tones of summed type. Further by introducing the conception of the apparent external force, the phase angle and the backbone curves can be obtained, and the effect of unsymmetrical nonlinearity and damping force can be explained clearly.