The effectiveness of the homogenization method for analysis of composite materials has been shown since it was firstly introduced in 1970's. As this method is applicable to periodic problems only, however, some engineering problems remain unsolved. For instance, we could not apply it to honeycomb sandwich plates and panels with reinforcing ribs because these structures are not periodic in the thickness direction. In this paper, the application of the homogenization method to those composite structures is discussed. As a practical example, stress analysis of single-layered corrugated core sandwich plate is presented. A method to homogenize the complicated geometry to a simple solid model, to get the homogenized material constants, and to calculate the microscopic stress is proposed.