An apparatus of the invention for inspecting bottles for detecting flaws or contamination in the bottles relies upon the light converging function performed by one open-sided elliptic cylinder having reflective inner surface and two foci. The point or line to be inspected of the bottle is positioned at one of the two foci, while at least one light receiving element is disposed at the other focus. A beam of light is applied to the point or line to be inspected of the bottle, so that, if there is any defect such as flaw or contamination, the light is reflected irregularly by the bottle in indefinite direction. The irregularly reflected fractions of light are reflected by the inner surface of the elliptic cylinder and converge on the light receiving element disposed at the other focus, so as to be effectively received by the same element. the presence of the defect, if any, is detected from the output of the light receiving element. Another light receiving element may be disposed outside of the elliptic cylinder so as to receive the fraction of light passed through the bottle.