To provide a delineator or a delineator installing device which is low in installation cost and very simple in installation work.

A reflecting member 3 is formed at its rear surface 5 with reflective concave and convex parts so as to obtain retroreflective light having a high intensity when the incident angle of light laterally incident upon the front surface 1 is in a range of 15° to 45°. The reflecting member 7 is fitted between side walls 23 of a cover member 7 with its rear surface 5 facing the cover member 7, and is secured thereto by an adhesive. A double surface adhesive tape 1 is stuck, and thereafter a releasable paper 27 is removed off therefrom. Then, the cover member 7 is stuck to the outer surface of a guard rail so that light from an automobile is retroreflected by the reflecting member 3, having a high intensity.