Coherent edge radiation (CER) is not well known light. However, by understanding the radiation characteristics, it is found that charm of the CER is not only that it is useful as an intense terahertz light source. Because the CER can be observed without damaging free-electron laser (FEL) oscillations, it can reveal the electron bunch shape immediately after FEL interaction. It was confirmed that the electron bunch shape was modulated by FEL oscillations by observing the CER.