An electrochromic device employing a metal oxide film as an oxidative-coloration layer is provided. The metal oxide film is a porous film comprising grains, 50% in number or more of which have a grain size of 1000 to 5000 .ANG. in diameter, and changes its optical density in a range of 0.3 to 0.65 between a bleached state and a colored state. The metal oxide film is prepared by an anodic oxidation process in which alternating potential is continuously applied to a metallic film initially with a smaller amplitude and subsequently with a larger amplitude.