In a lock-step synchronism fault-tolerant computer system including 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 detecting disagreement in a state of access to an external bus among the respective processors in each computing module, if no fault is detected in the system including each computing module, processing of resuming operation in synchronization is executed with respect to each computing module after generating an interruption to all the processors to execute delay adjustment for making a state of instruction execution among computing modules be coincident.