To provide a reinforcement member which can easily store a corner reinforcement piece in a packaging box body without hooking to a body part upper edge of a packaging box body, and to provide a packaging box using the same.SOLUTION: A reinforcement member 3 is to be stored in a packaging box body 2 which includes a body part constituted by a plurality of side plates 5, and a bottom part. It includes: a reinforcement bottom plate 11 having a pair of first side edges 11a, a pair of second side edges 11b and third side edges 11c, and stored in the packaging box body 2 and overlapped with a bottom plate 4; reinforcement side plates 12 which stand from the first side edges 11a of the reinforcement bottom plate 11, and stored in the packaging box body 2 and overlapped with the side plates 5; corner reinforcement pieces 13 continuously provided to the side edges of the reinforcement side plates 12; and guide pieces 14 continuously provided to lower end edges of the corner reinforcement pieces 13 and side edges of the reinforcement bottom plate 11, and overlapped with the reinforcement bottom plate 11. An outer edge of the guide piece extends so as to connect the side edge of the corner reinforcement piece and the second side edge of the reinforcement bottom plate.SELECTED DRAWING: Figure 4