There is provided a travel reference line determination system and an automatic driving system capable of determining a travel reference line of a vehicle appropriately even under conditions where information representing the track environment of the vehicle is hard to get. An ECU of an automatic driving system calculates a model y coordinate value ymw_i using a map in FIG. 4 (Step 12), calculates an estimated y coordinate value y_i using track environment data D_info (Step 11), calculates curvature C so that an error between the model y coordinate value ymw_i and the estimated y coordinate value y_i may be minimized (Step 18), calculates a travel trajectory Xf using the curvature C (Step 4), and executes automatic driving control using the travel trajectory Xf (Steps 31 to 33).