Resin transfer molding (RTM) process is getting popular for fabrications of complicated commercial products made from GFRP at low cost. For the RTM process of complicated components, it is indispensable to do trial and error over and over to find an optimal fabrication process. The trial process is time consuming and that is one of the reasons of the high cost of composites. To overcome this problem, it is useful to monitor the degree of cure of the target GFRP products. In the present study, the luminance change method is adopted for cure monitoring for the polyester-based composite. Polyester resin is usually adopted as matrix resin of the low-cost GFRP products. The polyester resin usually changes its optical property during the curing process. This enables us to monitor the degree of cure by means of the measurements of luminance change of the transmitted light or the reflected light. In this study, we use the system measured with the reflected light. The sensing system employs a LED as a light source and a photodiode as a light power sensor. This low-cost cure-monitoring system is applied to monitoring of the degree of cure of GFRP. The degree of cure is measured by means of commercially available dielectric sensors, and results are compared with the results of luminance change. The effectiveness is confirmed experimentally here.