Using the laser-ultrasonic visualizing technique, we attempt to detect the delamination of a honeycomb sandwich structure based on the change in dispersive property. By replacing the honeycomb core with an equivalent homogenized layer, a three-layer model of the honeycomb sandwich structure can be constructed. Then, the dispersion relation of the guided waves can be derived and it was found that the wavenumber of A₀ mode increases on the delamination region. Moreover, by scanning the laser on a specimen with artificial delamination, the guided waves were received by a PZT sensor. Then, the two-dimensional wavenumber map was derived by taking the ratio of Fourier spectrum between the adjoint data points. As a result, it is shown that the delamination can be detected based on the enhancement of the wavenumber. In addition, since the cell wall is clearly visible in the intact region on the wavenumber map, the delamination can be detected based on the disappearance of the cell wall.