A communication method is provided to reduce an overhead of inter-processor synchronization for a communication phase in collective communication and to speed up the collective communication. Each of processors in a parallel computer start a previous process before a collective communication phase in which communications are performed at a same time among the processors through a inter-processor network. Each processor executes a synchronization command in advance at a time when a portion of the previous process for a predetermined time t is left. The inter-processor synchronization control section transmits a synchronization completion notice to each processor, if a synchronization condition is met. For the period, each processor executes the previous process in parallel. Then, the plurality of processors enter the collective communication phase.