A branch-lane entry judging system includes a traffic-line detecting means for detecting traffic lines on right and left sides of a vehicle, a length and width detecting means for detecting lengths and widths of the traffic lines in a real space, a lane-type judging means for judging types of the traffic lines and a type of a lane in which the vehicle is running, a branch-lane detecting means for detecting a branch lane when one of the traffic lines changes from a solid line to a broken line having a predetermined length and a predetermined width, the branch lane existing near the changed traffic line, and a branch-lane entry judging means for judging that the vehicle will enter the branch lane when the vehicle moves toward the branch lane or a directional indicator is operated toward the branch lane while the branch lane is detected.