Screw fastening method and screw fastening device

A screw fastening device comprising:

• a shaft part that is engaged with an external thread or an internal thread and transmits rotational force;
• a motor that is able to drive the shaft part to rotate normally and reversely;
• a sensor configured to detect a collision between thread ridges the external thread and the internal thread, and
• a control part configured to control the motor, wherein
• the control part determines whether or not a time interval of collisions detected by the sensor matches a theoretical cycle of collisions, which is calculated from rotation speed of the shaft part, in a state where the shaft part, to which one of the external thread in a non-screwed state and the internal thread in a non-screwed state is engaged, is rotated reversely in a direction of loosening the thread engaged with the shaft part, and, when it is determined that the time interval matches the theoretical cycle, the control part controls the motor so that the shaft part is rotated normally to fasten the external thread and the internal thread to each other.