A screw capper 1 includes a capping head 6 which grips a cap 5, a servo motor 7 which rotates the capping head, a voltmeter 10 which measures the torque of the capping head, an encoder 11 which detects the rotational angle of the capping head, and a control device 8 which controls the servo motor. In the screw capper, whether rotational fastening is acceptable is to be determined during the rotational fastening, the measured value of the encoder is measured from a predetermined measurement starting point until a rotational fastening completion point, and if the measured rotational angle is in the range of an acceptable decision angle which is set in advance, it is determined that the rotational fastening has been acceptably performed, whereas if the measured rotational angle is not in the range of the acceptable decision angle, it is determined that the rotational fastening is unacceptable.In the screw capper, defective rotational fastening can be detected, and reductions in costs can be realized compared to a case where a device for detecting such defective rotational fastening is separately provided.