A travel assist system comprising: travel planning means for preparing, in the case where a vehicle travels along a predetermined travel route composed of links, a travel plan related to control of a drive motor and an engine used as a drive source of the vehicle for the travel route; congestion information acquisition means for acquiring congestion information on the travel route; characterised by vehicle state acquisition means for acquiring an operation mode of the vehicle, which is one of a long operation mode in which the travel plan is prepared for the entire travel route to control drive of the drive motor and the engine, and a middle operation mode in which the travel plan is prepared for only a range within a predetermined distance ahead of a position of the vehicle in a travel direction on the basis of a road status ahead of the vehicle in the travel direction to control drive of the drive motor and the engine, a communication load on communication within the vehicle, and a resource of a communication electronic control unit that controls communication within the vehicle as a state of the vehicle; variation determination means for determining, using a criterion based on the state of the vehicle, whether or not a congestion status of the travel route has been varied from the congestion information; and travel plan correction means for correcting the travel plan, in the case where it is determined that the congestion status of the travel route has been varied, using the congestion status after the variation, wherein the variation determination means is configured to calculate a congestion degree for each link using different criteria, which depend on the operation mode of the vehicle, the communication load, and the resource of the communication electronic control unit, and is configured to make a comparison between the calculated congestion degrees for each link and congestion information acquired one cycle earlier to determine whether or not the congestion status of the travel route has been varied. travel planning means for preparing, in the case where a vehicle travels along a predetermined travel route composed of links, a travel plan related to control of a drive motor and an engine used as a drive source of the vehicle for the travel route; congestion information acquisition means for acquiring congestion information on the travel route; characterised by vehicle state acquisition means for acquiring an operation mode of the vehicle, which is one of a long operation mode in which the travel plan is prepared for the entire travel route to control drive of the drive motor and the engine, and a middle operation mode in which the travel plan is prepared for only a range within a predetermined distance ahead of a position of the vehicle in a travel direction on the basis of a road status ahead of the vehicle in the travel direction to control drive of the drive motor and the engine, a communication load on communication within the vehicle, and a resource of a communication electronic control unit that controls communication within the vehicle as a state of the vehicle; variation determination means for determining, using a criterion based on the state of the vehicle, whether or not a congestion status of the travel route has been varied from the congestion information; and travel plan correction means for correcting the travel plan, in the case where it is determined that the congestion status of the travel route has been varied, using the congestion status after the variation, wherein the variation determination means is configured to calculate a congestion degree for each link using different criteria, which depend on the operation mode of the vehicle, the communication load, and the resource of the communication electronic control unit, and is configured to make a comparison between the calculated congestion degrees for each link and congestion information acquired one cycle earlier to determine whether or not the congestion status of the travel route has been varied. is configured to calculate a congestion degree for each link using different criteria, which depend on the operation mode of the vehicle, the communication load, and the resource of the communication electronic control unit, and is configured to make a comparison between the calculated congestion degrees for each link and congestion information acquired one cycle earlier to determine whether or not the congestion status of the travel route has been varied.