Honeycomb sandwich structures are widely used in aircraft owing to the superior characteristics, such as the light weight, the high specific bending stiffness and the high specific in-plane compressive strength. However the honeycomb sandwich structures are prone to have debonding damages at the interfaces between the skin and the honeycomb core, which degrades the mechanical properties largely. For inspection of damages in plate-like structures, the propagation of ultrasonic guided waves along the plate is effective. In this research, we attempted to detect the debonding at the skin/core interfaces in a honeycomb sandwich panel by using a laser ultrasonic visualization method. Debonding damages were artificially introduced in a sandwich panel consisting of two CFRP skin plates and an aluminum honeycomb core. Then, ultrasonic guided waves were excited in the panel through scanning of a laser irradiation on a surface of the plate and were received by a piezoelectric sensor installed on the same surface by using a laser ultrasonic visualizing inspector. As a result, we obtained visualization animations of the ultrasonic propagation behavior. From the change in the maximum amplitude distribution of the guided wave, we were able to identify the debonding damages at the skin/core interfaces in both the laser-irradiation side and the opposite side. Furthermore, a finite element analysis of the ultrasonic propagation in the honeycomb sandwich panel was conducted to confirm the phenomena observed in the experiments. From the calculation results, the mechanism of the observed phenomena was able to be clarified.