Mode selecting characteristics of four-layer dielectric slab waveguide have been studied, considering its practical application to the optical integrated circuits as a mode filter. The dimensions of the waveguides discussed here are restricted to the cases where only TE0 mode or TE0 and TM0 modes can exist and higher modes are in cutoff region. Cutoff characteristics and attenuation characteristics for TE0 and TM0 modes have been investigated for various waveguide parameters. It has been found from the results of this investigation that the four-layer waveguide can be utilized as a TE0 mode filter where only TE0 mode can propagate without attenuation, provided that the normalized frequency of the waveguide is chosen in the range between two cutoff normalized frequencies for TE0 and TM0 modes. In the latter part of this paper, the four-layer waveguide joined with a three-layer waveguide has been treated, and transmittance and reflectance for transmitted and reflected waves at the junction and also the fields of the transmitted wave in the neighborhood of the junction have been studied.