[Object] Provided are a “eutectic colloidal crystal” which is an aggregate of plural kinds of colloidal crystals having different lattice constants, a solidified body of the eutectic colloidal crystal, and methods for producing them.[Resolution means] The eutectic colloidal crystal of the present invention contains two or more kinds of colloidal crystals composed of substantially monodispersed colloidal particles having different particle sizes. This eutectic colloidal crystal is obtained by providing a colloidal dispersion of two or more kinds of colloidal particles having different particle sizes, and a polymer which will not substantially adsorb to the colloidal particles (the coefficient of variation in particle size of these colloidal particles is less than 20%) dissolved in a dispersion medium (dispersion preparation process), and allowing the colloidal dispersion to stand (eutectoid process).