A metal plate bending device, including a lower die having a main body including a pair of arc-shaped cross-sectional recesses formed on an upper surface in a symmetric design with respect to a center line, and swingable members swingably received in said recesses respectively. An upper die is arranged above said lower die movably along said center line relative to said lower die, wherein said upper die is moved toward a metal plate mounted on said lower die causing a press-push force to said metal plate to, causing said swingable members to swing and bend said metal plate along said center line. A movable plate is slideably in planar contact with a flat upper surface of the swingable members and is a flat supporting surface. During bending, said movable plate is slideable with respect to said swingable member, and is moved together with said metal plate.