A vehicle-mounted millimeter-wave communication device mounted on a vehicle and performing communication while temporally changing a communication direction by using an antenna unit capable of changing directivity, wherein the vehicle-mounted millimeter-wave-communication device is able to transmit a signal in only one direction at the time, the device comprising: an acquisition unit configured to acquire a relay request that requests relay of communication from a first communication partner; a detection unit configured to detect directions of the first communication partner and a second communication partner that are viewed from the vehicle, wherein the detection unit predicts the direction of the second communication partner at a future time based on a predicted position of the vehicle at said future time; and a communication control unit configured to: repeat a process of determining whether the condition that the relay request is received from the first communication partner and it is possible to communicate with the second communication partner, is satisfied, perform control such that, for each time it is determined that the condition is satisfied, communication is performed, in which a communication time longer than a communication time set for another direction is set for each of the directions of the first communication partner and the second communication partner, and communication between the first communication partner and the second communication partner is relayed, and wherein the communication control unit performs control such that, in the case where it is determined that the condition is satisfied, communication, in which a long communication time is set for each of the directions of the first communication partner and the second communication partner, and communication, in which communication times that are equal to each other are set for all directions, are performed alternately for a predetermined time, and then the process of and is repeated. an acquisition unit configured to acquire a relay request that requests relay of communication from a first communication partner; a detection unit configured to detect directions of the first communication partner and a second communication partner that are viewed from the vehicle, wherein the detection unit predicts the direction of the second communication partner at a future time based on a predicted position of the vehicle at said future time; and a communication control unit configured to: repeat a process of determining whether the condition that the relay request is received from the first communication partner and it is possible to communicate with the second communication partner, is satisfied, perform control such that, for each time it is determined that the condition is satisfied, communication is performed, in which a communication time longer than a communication time set for another direction is set for each of the directions of the first communication partner and the second communication partner, and communication between the first communication partner and the second communication partner is relayed, and repeat a process of determining whether the condition that the relay request is received from the first communication partner and it is possible to communicate with the second communication partner, is satisfied, perform control such that, for each time it is determined that the condition is satisfied, communication is performed, in which a communication time longer than a communication time set for another direction is set for each of the directions of the first communication partner and the second communication partner, and communication between the first communication partner and the second communication partner is relayed, and wherein the communication control unit performs control such that, in the case where it is determined that the condition is satisfied, communication, in which a long communication time is set for each of the directions of the first communication partner and the second communication partner, and communication, in which communication times that are equal to each other are set for all directions, are performed alternately for a predetermined time, and then the process of and is repeated.