Tensile stress field induced by fibroblasts in collagen gel are investigated by the cell culture method. Specimens of thin collagen gel membrane, within which cells proliferate, are subjected to various initial and boundary conditions, and cell stretching, cell orientation, cell migration, cell proliferation, induction of tensile stress field in collagen gel, and geometry transformation of specimens are observed. It is demonstrated that fibroblasts induce tension, change their orientation along the tensile direction, and create structures composed of collagen fibers. A hypothetical mechanism for such automorphogenesis is proposed wherein fibroblasts induce tension and produce tense collagen fibers. Cells then stretch along the tense fibers, which increases tension in this direction. Thus this mechanism functions as a positive feedback loop which enables cells to form structures with collagen fibers according to the mechanical environment.