An experiment on detecting an unbonded defect under the surface of a glass model using Rayleigh waves was executed. A 1 MHz Rayleigh wave was propagated on the glass model, and the vibration distribution on the material surface was probed by a laser beam. No information of the defect was detected from the Rayleigh wave amplitude distribution. On the other hand, the phase information of Rayleigh waves propagating on the material surface was focused on and acquired. As a result, the measured phase change for the propagation distance x (difference in phase slope) was detected in the unbonded defect area. In addition, finite element method (FEM) analysis was performed under the same conditions as in the experiment to verify the experimental results. A phase change was also observed on the unbonded defect, as in the experiment. The possibility of this method as a new detection method for the effective evaluation of unbonded defects in a material is described.