The sintering of advanced ceramics requires very fast sintering method in order to avoid both grain growth and inter-diffusion. In this context, the microwave sintering is a very powerful method since it enables sintering in a very short time. However, up to now the trial error method is usually used to find the best parameters (incident power, resonance mode, etc.) to obtain the expected microstructure. This method is time consuming and a high reproducibility cannot be achieved. Therefore, to control the sintering progress, only the in situ shrinkage measurement is required. In this work, a specific TE10m microwave cavity was designed to follow up the shrinkage during the microwave sintering using an optical-based position sensing device. The basic principle consists in measuring the distance from a laser source to the sample surface by means of a triangulation method. The spatial resolution device is enough to accurately measure the shrinkage versus time of a microwave irradiated sample. Shrinkage time curves were recorded during the direct microwave sintering of CuO chosen as a test material. The kinetic of sintering was found extraordinarily fast as, to achieve a satisfying density, only a few seconds are needed. This new method is undoubtedly attractive to increase our understanding of microwave sintering and very useful to control the microstructure of the microwave sintered ceramics.