The present invention comprises a plurality of processor modules and shared storage modules connected through a system bus. It improves system performance by limiting the range and shortening the time of exclusive control when a CSI (compare and store interlocked) instruction is executed to rewrite the contents of a shared storage if read data at an accaccaccess address of a shared storage coincide with the data anticipated by a CPU. In the present invention, when the CPU of the processor module rewrites data by addressing any shared storage module in executing a CSI instruction, the CPU has a connection unit connected through a system bus of a shared storage module recognize the CSI instruction, and reads data and compares them with comparison data in the shared storage module. Thus, the range of the exclusive control is limited to the process in the internal bus in the shared storage module.