A flexible workpiece assembling method for assembling a flexible workpiece (W) with an object (60) by using a first robot (20) comprising a first hand (21) and a second robot (30) comprising a second hand (31) includes the steps of: gripping a first portion of the flexible workpiece by the first hand and gripping a second portion of the flexible workpiece different from the first portion by the second hand; moving the flexible workpiece to the object through a cooperating operation of the first and second robots; deforming the flexible workpiece into a shape corresponding to the nonflexible object by the first and second robots; and assembling the flexible workpiece onto the nonflexible object by the first and second robots. As a result, the flexible workpiece can be accurately assembled onto the nonflexible object without breaking or tearing. The first and second robots may comprise a pushing means for pushing the flexible workpiece into the object.