A camera is used to take an image of an end surface of a honeycomb structure gripped by a hand. Based on the image, an initial rotation angle of the honeycomb structure around a vertical axis at a reference position where the image has been taken is recognized. An arm turning section is driven to convey the honeycomb structure gripped by the hand from the reference position to above a plugging mask. A rotation angle required to adjust the rotation angle of the honeycomb structure on the plugging mask to a desired final rotation angle is acquired based on the initial rotation angle recognized at the reference position and a rotation angle of the honeycomb structure around the vertical axis associated with the driving of the arm turning section from the reference position to above the plugging mask. The honeycomb structure is rotated based on the required rotation angle.