PURPOSE: To make an apparatus simple and inexpensive, by constituting so that a light diffusion means diffusing and passing through incident light is included and this means is inserted into an optical axis of a main illuminating system crossing nearly vertically.

CONSTITUTION: A diffuser for spot magnification 60 is provided between a light shading plate 40 and a condenser lens 50 and is inserted into an optical axis of a main illuminating system crossing nearly vertically as shown by an arrow mark E. On this occasion, the diffuser 60 is made of flat board sand scattered glass etc. and incident light from its one hand face is diffused and is transmitted to the other face by the diffuser 6. An auxiliary light source or lens for spot magnification for illumination of monitoring is not used for this optical measuring apparatus and construction of the apparatus is simplified by using the simple diffuser, and also the price of the apparatus is reduced.