The interference spectrum characteristic by Gaussian beam irradiated to two dimensional trench of the dielectric is examined by the boundary element method. The interference spectrum data is obtained by changing the permittivity, the trench depth and the trench width, and the polarization of the beam. Then, the trench depth is examined by the interference spectrum data using the continuous wavelet method. Using this new method instead of the FFT processing, the depth is accurately measured at the cutoff width because the wavelet has the frequency and time information as well.