An extension tool is provided for stretching a sheet-shaped material over its entirety with minimal adjusting portions and uniform tension. Four corner members are disposed along with four frame members for forming a rectangular frame. Four rods are disposed so as to pass through the four frame members and allow the sides and corners to move towards and away from each other for adjusting the size of the frame. One set of corner members are located on the diagonals and are provided with a moving mechanism for moving the rods along the sides of the rectangle. The extension tool for tensioning the material further comprises a transmission mechanism for converting the movement of a rod in a first side of the rectangle between first corner pairs into a reversing and approaching movement of a second rod in a second side of the frame between second corner pairs.