A packet communication device autonomously selects an appropriate operation mode according to a connection environment to an external device before a service of the device is started. When the device is connected to the external buses, connection interface units notify an external device discrimination unit of connection of the device. The external device discrimination unit issues a polling packet to the connected device, discriminates the connected external device on the basis of the response packet, and notifies an operation mode switching unit. The operation mode switching unit selects an operation mode conforming to a connection environment of the packet communication device to the external device and switches the operation mode of the device to the mode. The selection of the operation modes is completed before the entire device or a portion of the device which is influenced by the selection of the operation mode starts a service to the external device to which the packet communication device is connected.