A computer assisted teaching system which enables sharing the same operation with a large number of users using P2P is shown. The users of this system can share the same operation of applications such like a text editor, a draw program and others, on their computer terminals in real-time. This system can show an operation on a terminal to all terminals in the latency of O(log N) time complexity, where N is the number of terminals. In order to share the same operation on computer terminals, at most one operation must be performed on them at a time. In order to realize this, a mutual exclusion algorithm is embedded in this system. The time complexity of entering the critical section is O(log N). We have compared the performance of this system with the performance of a client-server system. A game could be played by remote users using this system.