Abstract: Disclosed is an apparatus for measuring surface stress of glass coating and transparent plastic product, which comprises means for projecting the coherent light on the surface of the object to be measured, an output prism having a refractive index greater than that of said surface, mounted proximal to the light incident point on said surface in such a manner that one face thereof is in optically close contact with said surface, a telescope to measure the critical angle at the interface of said surface and said output prism in respect of the projected light from said output prism, and a polarizer located on the optical path of said projected light. The apparatus facilitates easily a direct and non-destructive measurement of the surface stress without any complex calculation.