The authors developed a damage detection system that generates ultrasonic waves with a piezo-ceramic actuator and receives them by a fiber Bragg grating (FBG) sensor. In this research, this system was applied to evaluate debonding progress in CFRP skin/stringer bonded structures. FBG sensors were bonded on the stringer or embedded in the adhesive layer. Then, ultrasonic wave at 300kHz was propagated through the debonded region, and the wavelet transform was applied to the received waveform. After that, a new damage indexand a correlation coefficient were calculated from the distribution of the wavelet transform coefficient. As a result, the damage index increased and the correlation coefficient decreased with an increase in the debonded area. Hence the length of the debonding between the skin and the stringer could be well evaluated.