An extension tool of a sheet-shaped material comprising, in order to stretch a sheet-shaped material over its entirety at minimum adjusting portions and with a uniform tension, four corner members disposed at locations corresponding to the four corners of a rectangle, four frames disposed at locations corresponding to the sides of the rectangle and supported at the opposite ends thereof by the corner members so as to be movable in the directions of the sides, and four rods disposed so as to pass through the four frames respectively. One set of corner members located on the diagonals of the rectangle are respectively provided with moving mechanism for moving the rods along the sides of the rectangle. The extension tool of a sheet-shaped material further comprises a transmission mechanism for converting the moving of a rod on a side of the rectangle in a direction from one corner member toward the other corner member making a pair with the one corner member (or its reverse direction) into the moving of the other rod stretching from the other corner member in a direction away from the other corner member (or approaching direction).