Simultaneous structural optimization procedure was proposed to improve the critical dynamic pressure for supersonic flutter and to reduce the structural mass of plate wing using Magnesium alloys with an optimization procedure based on Evolutionary Structural Optimization. The distribution of bending stiffness of the wing, that was represented by the distribution of the equivalent thickness of the plate wing, was considered for the design parameter. The mass density which was proportional to the equivalent thickness was considered in the optimization procedures. Two types of sensitivity parameters were introduced to perform the simultaneous structural optimization, i.s. a sensitivity of the difference of the eigen-frequencies between the first and the second flutter modes and that of the tip wing deformation due to the change of the equivalent thickness of the FEM element. Simultaneous structural optimization procedure based on the Evolutionary Structural Optimization was formulated by considering the two types of sensitivities. A delta wing model with Magnesium alloy was used for the numerical demonstration to examine the performance of the optimization procedure and the properties of the optimum design. The numerical results indicated that the optimum design with Magnesium alloys had the better performance without conflicting of the critical dynamic pressure and structural mass under a given constraint for the tip deformation. Also, the effects of mass and stiffness distribution on the optimum design were clarified.