An external force application unit is allowed to impact on a sheet material, and an external force detection unit detects an impact force through the sheet material. Further, there is provided a push-in sensing unit for detecting abnormality of the sheet material entering a space between the external force application unit and the external force detection unit. When the push-in sensing unit detects the abnormality, a control circuit operates a motor to rotate a cam, thereby moving the external force application unit to be retracted in a direction in which the external force application unit is moved away from an entering position of the sheet material.