This paper proposes a web-based online lecture system that uses a user operation synchronization method. Online lecture is a kind of popular style of lecture. To provide a real-time lecture, a teacher shares own screen with students as a video stream. However, the video stream requires a large amount of network bandwidth. Some students have only mobile network environment. In Japan, the monthly data charges for mobile network is limited. Therefore, the online lecture with video stream have a problem for these students. To solve this problem, our study proposes a web-based online lecture system. This system shares the teacher’s operation as a JavaScript event object. The screen on the student side is updated by the received event object. Using this mechanism, we can reduce the data amount to share the lecture contents.