A communication device is connected to first and second networks. The communication device includes a processor executing a process including relaying communication between the first and second networks; and confirming connection to a first device connected to the first network or a second device connected to the second network, based on a device list registered in advance. The confirming includes blocking the relaying when the confirmation of the connection to the first device is unsuccessful; enabling the relaying when the confirmation of the connection to the first device is successful and when the second device is not registered in the device list; enabling the relaying when the confirmation of the connections to the first and second devices is successful; and blocking the relaying when the confirmation of the connection to the first device is successful and when the confirmation of the connection to the second device is unsuccessful.