A sensor system for inspecting the layer surface quality in stereolithography process is proposed in this paper. Since stereolithography process builds 3D shape by forming layers repeatedly, it is very important to process each layer of stereolithography process products in some favored conditions: Every layer should be cured uniformly and hardly enough so that the adjacent two layers can stick together to each other. However, in many applications, two kind of defects are frequently found, i.e. void and delamination. Void is cavity inside the built part and delamination is detachment of the bond between two adjacent layers. To inspect such defects, we propose a sensor system which consists of a laser source, a galvanometer scanner, a photo- detector, a few lenses, and a beam splitter. In this sensor system, the laser beam and the field of view of the detector are co-axially positioned and scanned over the product surface by the galvanometer. The reflected light is then detected by the photo-detector. And from the photo-detector signal, the surface condition and quality of the layer being inspected can be estimated. Since stereolithography products are very transparent, the system needs very fine tuning of the system parameters that include the power of laser beam and the sensitivity of the detector, and etc. The experimental results are obtained for products of a variety of shapes and several cases are presented and discussed in detail.