According to one embodiment, a communication device belongs to a communication network including a control device and a plurality of communication devices connected to the control device, and transmits a communication packet to a transmission destination communication device. The communication device and the transmission destination communication device are differently one of the plurality of communication devices. In the communication device, a memory stores first information for judging a normality of the communication packet. An analyzing unit judges the normality of a received communication packet based on the received communication packet and the first information. A transmission destination determining unit determines the transmission destination communication device and the control device as transmission destinations of the received communication packet when the analyzing unit judges that the received communication packet is not normal. A generating unit encrypts the communication packet to be transmitted to the transmission destinations.