PROBLEM TO BE SOLVED: To provide a lightweight plant cultivation box, easy to assemble, capable of absorbing the unevennesses of the installation surface, causing no growth of plant roots outward from the bottom face, and suitably usable for greening a concrete construct as a building rooftop. SOLUTION: This plant cultivation box has a square bottom face and four side faces; wherein the cultivation box is formed of a one-piece plastic sheet; each of the side face sheets is perpendicularly bent upward at the boundary line between the bottom face and the side face to form an upward side face; the side face sheet is folded back inward of the box at the line of the box height to form a downward side face; drain holes are made at the upward side faces; each of joint flaps provided at the respective side ends of the upward side faces is held by the corresponding upward side face and downward side face which are adjacent to each other to maintain the objective box to be in a box shape.
