We applied magnesium-titanium (Mg-Ti) thin film as the optical switching layer to all-solid-state switchable mirror on plastic sheet (polyethylene terephthalate, PET) in the viewpoint of clear transparency at the transparent state. For the switching speed from the reflective to the transparent states, the PET-device showed a little slower switching speed than the glass-device, and it depended on the sheet resistance of indium tin oxide (ITO). When Mg-Ti thin film was applied to the PET-device, absorption at the visible ray was reduced, resulting in near-colorless state at the transparent state. The PET-device with Mg-Ti thin film showed chromaticity coordinates of x = 0.341 and y = 0.339 and a luminous transmittance of 42.4% at the transparent state. However, the PET-device had lower durability than that of the glass-device. It seemed to be related with the degradation mechanism of the optical switching layer. When Mg-Ti thin film after the durability test was analyzed by XPS, though the glass-device had much oxidized state of magnesium, the PET-device had mixture states of oxide and hydroxide in the optical switching layer. We suggest that the hydroxide in the layer will be formed by including water in the PET sheet, and the hydroxide might significantly affect the rapid degradation of the PET-device. (C) 2009 Elsevier B.V. All rights reserved.