In the present paper, we propose a method for in-air knotting by a dual-arm, multi-finger robot. Most previous studies about knotting by robots assume that the rope is placed on a table. However, in-air knotting requires more skillful hand manipulations, such as swapping ropes between the right and left hands. Therefore, we herein focus on hand motion and first extract the essential hand motions for knotting, referred to herein as skill motions, by observing human knotting procedures. Next, hardware with the capability of executing these skill motions is developed. Finally, experiments are conducted to confirm that several types of knots can be tied in the air using the proposed method.