An image forming device (2) has a first connector (3) and a second connector (4) for connecting an internally disposed printer engine (1) to a first external device (5) and a second external device (6), and a control unit (7) having an alternate operation mode in which communication with the first external device (5) and second external device (6) is performed alternatingly by changing over communication at a predetermined cycle. During communication with either one of the first and second external devices (5, 6), the control unit (7) enables reception of commands and/or a print-start signal from this external device and refuses reception of commands and a print-start signal from the other external device. When communication is changed over from one external device to the other 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.