An image forming device for connection to first and second external devices, including a control unit having an alternate operation mode for performing alternating communication with external devices by switching communication at a predetermined cycle. During communication with one of such external devices, the control unit enables reception of commands and/or a print-start signal from that external device and refuses reception of commands and a print-start signal from another external device. When communication is changed over from one external device to another external device, the control unit provides an interval in which reception of commands and a print-start signal from both external devices is refused for a predetermined period of time.