the copier a becomes capable of receiving simulation data from the controller b it first sends a busy signal to the controller b step n4 reception of this busy signal causes the controller b to send simulation data generated by the control unit 22 to the copier a step n5 reception of this simulation data by the copier a causes the control unit 2 to execute the simulation the control unit 2 in which a program for executing the simulation has been previously incorporated executes the simulation based on the simulation data it receives step n5 when the simulation is completed the simulation or diagnostic results are loaded in a transmission buffer not shown in the control unit 2 and an end signal is sent to the controller b step n7 after which the simulation results edited in the transmission buffer are sent to the controller b through the modem 11 the duplexer 12 and the telephone line 9 step n8 when the simulation results are received at the controller b an end signal is sent to the copier a step n9 reception of the end signal at the copier a causes the duplexer 12 to connect the telephone line 9 to the telephone 10 step n1o and the simulation mode in the copier a is completed when the simulation results from the copier a are received at the controller b the repairman determines the cause of the malfunction based on the simulation.