Resin transfer molding (RTM) process is getting popular for fabrications of complicated commercial products made from Glass Fiber Reinforced Polymer (GFRP) at low cost. Cure monitoring is indispensable for RTM process. In the present study, polyester resin is adopted for matrix resin of the GFRP components. The polyester is usually adopted as matrix resin of the low-cost GFRP products. Existing methods for the cure monitoring are, however, expensive for the GFRP products. The polyester resin usually changes its optical property during curing. This enables us to monitor the degree of cure by means of measurements of luminance change of the transmitted light. Since the electrical circuit for measuring luminance change is not expensive, this system utilizes the luminance change for monitoring cure is not expensive system. In the present study, the sensing system employs a LED as a light source and plastic optical fibers as light paths. A photodiode is adopted as a light power sensor. This low-cost cure-monitoring system is applied to monitoring of degree of cure of polyester resin. Degree of cure is measured by means of commercially available dielectric sensors, and results are compared with the results of luminance change. The effectiveness of the method is confirmed experimentally here.