To power-assist a person in opening/closing a folding door by detecting the person's trying to open/close the folding door on the side of an actuator for opening/closing the door without installing a special detector on the door side.

A folding door type swing has a first door panel 1 horizontally rotated with one side edge rotatably supported to one side of a door frame 14; a second door panel 2 with one side edge rotatably connected to the other side edge of the first door panel 1; and an actuator for applying rotational force to the first door panel 1 to operate an opening part of the door frame 14 to open/close with the door panels 1, 2. The opening/closing device for the automatic door has a rotation detecting means for detecting the rotation of the first door panel 1, and the actuator drives to rotate the first door panel 1 in the rotating direction when the rotation detecting means detects slight rotation of the first door panel 1.