PURPOSE: To make it unnecessary to add hardware by checking the state of a transmission and a reception data lines by collecting the status of a peripheral device in a short period or a long period according to the stopped state or the running state of the peripheral device.

CONSTITUTION: When power is charged to each peripheral device 3 to 8 by a signal from a control device 2 to make it execute necessary operation, a power-on signal (PI), a transmission possible signal (CS) and the status (a) are sent from the corresponding peripheral device 3 to 8 to the control device 2, and the running state of the corresponding peripheral device is checked. Then, if the change of the running state of the corresponding peripheral device occurs, the status (b) is sent to the control device. Thus, the state of the lines of transmission data SD and the reception data RD can be always monitored, and the latest status of picture forming system can be informed from the control part 2 to a host machine 1.