Appropriate communication suitable for various situations is performed in millimeter wave communication for vehicles. 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 includes: a running environment acquisition unit configured to acquire a running environment including at least one of a road condition of a road on which the vehicle is running and a vehicle state of the vehicle; and a communication control unit configured to perform control such that communication, in which a communication time allocated to each communication direction is determined in accordance with the running environment, is performed.