Even when a tapping screw is used in a soft material, it is determined whether or not a screw tightening defect has occurred. A PLC (10) comprises a determination unit (13) which for a rotational torque data sequence having a predetermined number of data points, generates a normalized torque data sequence that is the amount of change with respect to rotational torque at a first data point of the data sequence, and determines whether screw tightening is defective, on the basis of the normalized torque data sequence.