To provide an apparatus for measuring the thickness of the glass layer of the sidewall of a glass container.

A glass container 22 is put in an oil bath 26 whose refractive index coincides with that of the external surface of the sidewall of the container. The container body is dipped in the bath so as to be rotatable with its lateral movement restricted, while the finished part of the container is rotated. Optical energy is directed into the bath and in the tangential direction passing the sidewall of the container 22, and generates an optical energy pattern as a function of the layer thickness of the sidewall. The thickness of the layer is determined as a function of an optical energy pattern like this. When this method is applied especially to a case of measuring the casing glass layer thickness of the casing glass container 22, a casing glass thickness like this is determined as a function of the pattern of optical energy outgoing from the oil bath.