An external connection device is connectable to a host device. The host device includes transmission means for transmitting, to the external connection device, a parameter and an operation control command by issuing a transfer protocol command (TPC). The external connection device includes a parameter register for storing the parameter, a command register for storing the operation control command provided from the host device, and a controller for referring to the operation control command stored in the command register and for performing operation control specified by the operation control command. The parameter register and the command register of the external connection device are selectively accessed from the host device based on a memory access transfer protocol command (TPC) and a security access transfer protocol command (TPC) issued from the host device.