A controller (1, 41) comprising: a determining unit configured to determine whether or not a communication target device (32, 42, 52) with which communication is to be performed is a device (42, 52) that supports device authentication using an electronic certificate (122, 222); wherein the controller (1, 41) is configured to perform encrypted communication with the communication target device which supports and succeeded in the device authentication using an electronic certificate (122, 222); characterized by the controller (1, 41) further comprising: a functional restriction unit configured to impose restrictions on the operation of on one or more functions of the communication target device (32, 52), when the determining unit determines the communication target device (32, 52) does not support the device authentication using an electronic certificate (122, 222); and a communication unit configured to perform communication in plain text with the communication target device (32, 52) when operating a non-restricted function of the communication target device (32, 52) while the restriction is imposed by the functional restriction unit; wherein the determining unit is further configured to determine whether the communication target device is an unauthorized device or an authorized device associated with a higher level of security than the unauthorized device, when the determining unit determines the communication target device (32, 52) does not support the device authentication using an electronic certificate (122, 222), and wherein the level of the restriction imposed on an authorized device by the functional restriction unit is less strict than the level of restriction imposed on unauthorized device. a determining unit configured to determine whether or not a communication target device (32, 42, 52) with which communication is to be performed is a device (42, 52) that supports device authentication using an electronic certificate (122, 222); wherein the controller (1, 41) is configured to perform encrypted communication with the communication target device which supports and succeeded in the device authentication using an electronic certificate (122, 222); characterized by the controller (1, 41) further comprising: a functional restriction unit configured to impose restrictions on the operation of on one or more functions of the communication target device (32, 52), when the determining unit determines the communication target device (32, 52) does not support the device authentication using an electronic certificate (122, 222); and a communication unit configured to perform communication in plain text with the communication target device (32, 52) when operating a non-restricted function of the communication target device (32, 52) while the restriction is imposed by the functional restriction unit; wherein the determining unit is further configured to determine whether the communication target device is an unauthorized device or an authorized device associated with a higher level of security than the unauthorized device, when the determining unit determines the communication target device (32, 52) does not support the device authentication using an electronic certificate (122, 222), and wherein the level of the restriction imposed on an authorized device by the functional restriction unit is less strict than the level of restriction imposed on unauthorized device. a functional restriction unit configured to impose restrictions on the operation of on one or more functions of the communication target device (32, 52), when the determining unit determines the communication target device (32, 52) does not support the device authentication using an electronic certificate (122, 222); and a communication unit configured to perform communication in plain text with the communication target device (32, 52) when operating a non-restricted function of the communication target device (32, 52) while the restriction is imposed by the functional restriction unit; wherein the determining unit is further configured to determine whether the communication target device is an unauthorized device or an authorized device associated with a higher level of security than the unauthorized device, when the determining unit determines the communication target device (32, 52) does not support the device authentication using an electronic certificate (122, 222), and wherein the level of the restriction imposed on an authorized device by the functional restriction unit is less strict than the level of restriction imposed on unauthorized device.