A 60-GHz band corporate-feed waveguide slot array has been fabricated by diffusion bonding of laminated thin metal plates. Since a degenerated aperture with a height of 5.4 mm is used for the circularly polarized wave radiator, the number of thin plates with a thickness of 0.2 mm to be bonded is 27, which is a large number. Therefore, two layers of degenerated apertures with a thickness of 0.5 mm are placed with air layers to reduce the number of the plates and to avoid bonding. However, the frequency dependences of the reflection and axial ratio are narrower than those of the previous antenna. In this report, a concept for obtaining broadband frequency dependence using a small number of thin plates is presented. Formerly designed by HFSS (FEM), a fast method of method of moments with genetic algorithm will be used for broadband design.