Air-water swirling flows in a one-fifth model of a steam separator in a boiling water nuclear reactor are measured to obtain a database for modeling and verification of numerical methods for predicting swirling flows in the separator. Flow patterns, liquid film thicknesses, separated flow rates and the ratio W_s^* of the separated flow to the total liquid flow are measured using a high-speed camera, a laser focus displacement meter and flowmeters. Main conclusions obtained are as follows: (1) liquid transfer from droplets to liquid film is caused not only by droplet deposition but also by the collection of droplets on the vanes of the swirler, (2) W_s^* increases with the gas volume flux J_G and does not depend on the liquid volume flux J_L so much because a large centrifugal force caused by the swirler makes most of droplets in the gas core deposit on the liquid film before the separation and (3) a local peak appears in the axial distribution of film thickness, the position of which corresponds to the location where the droplet deposition caused by the centrifugal force has completed.