A lock-step synchronism fault-tolerant computer system includes a plurality of computing modules having a processor and a memory in which each computing module processes the same instruction string in synchronization with each other. When disagreement in a state of access to an external bus among the respective processors in each computing module is detected, if no fault is detected in the system including the respective computing modules, an interruption is notified to all of said processors. Synchronization among each computing module is recovered by adjusting timing of a response to an access which each processor executes by an interruption.