A reflector which is formed of three or more of reflector modules (called stacks) arranged in arrays and bonded to each other, the stacks being arranged to average the distribution of thermal deformations thereof in terms of inhomogeneity among the stacks in average coefficient of linear expansion, thereby suppressing deformation and deterioration of image-forming performance quality of the reflector under varying temperature conditions. The stacks are arranged in order of the magnitude of values of average coefficients of linear expansion possessed by said stacks so that a thermal deformation pattern produced is a relatively simple pattern, there is provided a reflector device in which correction of thermal deformation can be made by a few actuators.