A driving support ECU comprises: a roadside object detection section for detecting, in a case where a lane marking WR on at least one of the right side and left side is detected by a white line detection section and another lane marking WL is not detected, a roadside object GL on the other of the right side and left side; a white line estimation part for estimating a position of a lane marking VL1 on the other of the right side and the left side based on the detected roadside object GL; and an information outputting section for determining a departure from the lane in which the vehicle VC is running, based on the estimated position of the lane marking VL1 on the other of the right side and left side.