PURPOSE: To make it unnecessary to provide a control signal line between subsystems, by using in common a regular data transfer line between the subsystems, and the transfer line of the system control signal, in a compound computer system.

CONSTITUTION: The control subsystem SS stores in the RAM11 the information for controlling other subsystem SS, and the data channel unit 12 sends it out to the data highway station 2 through the information transfer line 4 in the same way as the conventional data transition. The station 2 sends out this information onto the data highway 3. On the other hand, in the subsystem SS to be controlled, the command circuit 21 discrimenates the control signal from the control subsystem, and controls through the control signal line 5 the CPU stop signal line 19, the CPU start signal line 20, the electric power source closing line, etc. As a result, in the subsystem to be controlled, start/stop of the CPU, close/cut-off of the electric power source, etc. are executed. In this way, not only a regular data but also a control signal are sent out onto the highway 3, and the transfer line is used in common.