1. A method for detecting a narrow road, comprising: receiving, from a distance measuring sensor, distance information of points on a ground line at which a vertical surface in a driving direction of vehicle meets a ground; calculating a relative height of the points from the distance measuring sensor to the ground based on the distance information; acquiring a left boundary point and a right boundary point at which a difference in relative heights from adjacent points among the points becomes maximal; acquiring a road boundary line of a driving road of the vehicle based on the left boundary point and the right boundary point depending on a movement trajectory of the vehicle when a difference in relative heights at the left boundary point and the right boundary point is more than or equal to a reference value; calculating a road width of the driving road based on the acquired road boundary line of the driving road; and determining that the driving road of the vehicle is a narrow road when the road width of the driving road is less than or equal to a reference road width.