PURPOSE: To make it possible to endure sufficiently the impact due to shaking of a vehicle even if the number of piled-up reflective plates is increased considerably by forming a detaining step part on the outside circumferential wall face projecting a wall, which is so formed that inside circumferential wall surfaces facing each other are parallel with each other, on the circumferential edge part of the parabolic reflective plate in the direction opposite to the reflective surface of this reflective plate.

CONSTITUTION: A wall R4 of a reflective plate R is projected on the circumferential edge of the rear face of a substrate R2 so that inside surfaces 11 and 11 facing each other in the diametric direction are parallel with each other as shown in the figure, and a detaining step part 13 is formed on an outside circumferential face 12 of the wall R4; and when reflective plates R are piled up on a pellet 8 as shown in the figure, an edge 14 of a wall R'4 of a reflective plate R, in the lower stage is engaged with a step part 15 formed on the outside circumferential face of a wall R"4 of a reflective plate R" in the upper stage. In this case, the wall R4 is formed with such height H that an attaching seat R3 formed on the rear face of the reflective plate in the lower stage is not brought into contact with a reflective surface R1 of the reflective plate in the upper stage when they are piled up.