For chip-to-chip inter-connection, a Tbit/s-class capacity is expected in the near future. Therefore, optical chip-to-chip interconnection has attracted considerable attention owing to its high-speed and power-saving operation. To achieve a huge capacity, a multilayer optical waveguide structure for connection between chips is indispensable, and such a structure requires vertical waveguide section which is much difficult to form than a conventional horizontal waveguide. For easy fabrication of the vertical waveguide section, a design of multilayer waveguide structure with nonuniform cuboid cores has been studied in our lab. However, an unnatural layer-number dependence of coupling efficiency has been observed.  In this year, we have found the reason of such an unnatural characteristics of coupling efficiency by detailed analysis on the light beam distribution and optimized the design of nonuniform cuboid cores. The optimized design can provide a ten-layer waveguide with a coupling efficiency larger than -4 dB. In addition, the crosstalk dependence on the channel pitch between two waveguides have been investigated. The results indicate that the nonuniform cuboid core structure can accept the 28% narrower channel pitch than the uniform cube core structure when the crosstalk criterion is -20 dB.