The formation mechanism of the factory-roof pattern in a circumferential notched specimen was examined. To achieve observation of the material's cracks internally, plastic material, that was acrylic, was used for the test. Cyclic torsional tests were performed with and without application of static tension. A factory-roof pattern was formed in the fracture surface when cyclic torsion tests were conducted with static tension. Formation of the factory-roof pattern was found to be dependent on testing conditions. When such a pattern was formed, many small cracks were initiated by shear mode, followed by their coalescence. The new cracks' initiation at the front of the shear mode cracks could be one of the reasons for the coalescence. After the coalescence and branching of cracks, the cracks grew under the control of the maximum principal stress criterion. This behavior was examined by direct observation of the cracks. The final fracture pattern showed the initiation of many shear-mode cracks and their succeeding coalescence. From those observations, we considered the formation of a factory-roof pattern model.