Abstract: The coupling device for securing adjacent boards to construct a display case is molded from a plastic material and made in such a way, that a pair of plate members are spaced from each other to define a groove for receiving a corner of the board therein. The pair of plate members are connected by a pair of triangular prismatic portions extending on two adjacent sides thereof and with the two adjacent sides formed into a right angle, the groove is positioned opposite to the two adjacent sides. Along the longitudinal direction of the triangular prismatic portion a slot or a rod is alternatively provided, and a connecting member is provided with at least two complementary parts thereon for mating with the rods or the slots of at least two other triangular prismatic portions of two adjacent coupling devices, so as to connect the same.