PURPOSE: To prepare the titled substrate coated with silicon oxide having high stability against saline water and high-temperature steam, by forming a silicon oxide coating film containing ZrO2 on the surface of a substrate.

CONSTITUTION: An alcohol solution containing 5W30wt% organic silicon compound (e.g. ethyl silicate) is added with a zirconium compound[e.g. ZrCl2, Zr(NO3)4, etc.]in an amount of 1W30mol% (as ZrO2), and the pH of the mixture is adjusted to 2W6 with an acid such as hydrochloric acid, acetic acid, etc. to obtain a film-forming solution. The solution is applied to the surface of a substrate made of glass, ceramic, plastic, etc., dried by heating, and baked in an oxidative atmosphere or in air at 300W650°C to obtain a silicon oxide coating film having a thickness of 500W2,000. The silicon oxide coating film thus obtained is resistant to the formation of spot defects and fine cracks even by the outdoor exposure.