To provide a method and apparatus for measuring the width of road shoulder, safely estimating a distance to a road shoulder or to a lane marking without interfering with driving during travelling in order to solve the following problem: when a driver looks at a door mirror or a monitor screen for grasping a hidden road shoulder or lane marking, the driver moves his or her line of sight largely and does not gaze forward to cause danger.

In the road shoulder estimating apparatus, a reference body for estimating a distance to a road shoulder which is invisible from a driver of an automobile or the distance to a hidden lane marking is installed on the inner surface of a windshield of the automobile, to allow a driver to estimate the distance to the hidden road shoulder and the distance to the hidden lane marking.