PURPOSE: To synchronize computer systems at high speed by giving information on a synchronism request by means of interruption and recognizing synchronism completion data which is to set the completion of synchronism in a shared memory in a composite computer system provided with the shared memory.

CONSTITUTION: The composite computer system 10 consists of a synchronism request system 11, an objective system of synchronism 12, the shared memory 21 and the like. When the synchronism request system 11 gives the synchronism request to the objective system of synchronism 12, it issues an inter-system communication instruction 14 to a signal generation means 18. The signal genera tion means 19 of an interruption generation means 17 gives information on the synchronism request to the objective system of synchronism 12 by interrup tion. The objective system of synchronism 12 sets synchronism completion data showing the completion of synchronism in a conversation area 22 by a synchro nism completion informing means 16. The synchronism completion supervisory means 15 of the synchronism request system 11 supervises the conversation area 22 and recognizes it to be the completion of synchronism when synchronism completion data is set.