A heat exchanger (23) includes a plurality of principal heat exchange sections (51a-51c) and auxiliary heat exchange sections (52a-52c). A first header-collecting pipe (60) includes an upper space (61) communicating with flat tubes (33) of all of the principal heat exchange sections (51a-51c). Each of the auxiliary heat exchange sections (52a-52c) is in series connection to a corresponding one of the principal heat exchange sections (51a-51c). Of tube number ratios of the number of the flat tubes (33) constituting each of the heat exchange sections (51a-51c) to the number of the flat tubes (33) constituting a corresponding one of the auxiliary heat exchange sections (52a-52c), the first principal heat exchange sections (51a) which is the lowermost one has the smallest tube number ratio. Consequently, discharge of liquid refrigerant from a lower portion of the first principal heat exchange section (51a) is accelerated during defrosting, thereby shortening the time required for defrosting.