A heat exchanger comprising: a plurality of flat tubes; a first header-collecting pipe connected to an end of each of the flat tubes; a second header-collecting pipe connected to the other end of each of the flat tubes; and a plurality of fins joined to the flat tubes, the heat exchanger provided in a refrigerant circuit which is configured to perform a refrigerating cycle, and causing a refrigerant to exchange heat with air, wherein the first header-collecting pipe and the second header-collecting pipe are in an upright position, a plurality of heat exchange sections (51a-51c) each of which is constituted by adjacent flat tubes arranged one above the other, the first header-collecting pipe includes therein one communicating space which communicates with the flat tubes of all of the heat exchange sections (51a-51c), the second header-collecting pipe includes therein subspaces (71a-71c) which correspond to the heat exchange sections (51a-51c) on a one-by-one basis and each communicate with the flat tubes constituting a corresponding one of the heat exchange sections (51a-51c), auxiliary heat exchange sections (52a-52c) constituted by flat tubes, which auxiliary heat exchange sections correspond to the heat exchange sections (51a-51c) on a one-by-one basis, the flat tubes constituting the auxiliary heat exchange sections (52a-52c) are smaller in number than the flat tubes constituting the heat exchange sections (51a-51c), and the auxiliary heat exchange sections (52a-52c) are each in series connection to a corresponding one of the heat exchange sections (51a-51c), characterized in that the heat exchanger further includes a discharge accelerator which accelerates discharge of the refrigerant in a liquid state from a lower portion of the heat exchange section which is the lowermost heat exchange section during defrosting in which the refrigerant in a high-pressure gas state is introduced from the communicating space to the flat tubes in order to melt frost having formed on the fins, tube number ratios are obtained by dividing the number of the flat tubes constituting each of the heat exchange sections (51a -51c) by the number of the flat tubes constituting a corresponding one of the auxiliary heat exchange sections (52a-52c), the tube number ratio of the heat exchange section that is the lowermost heat exchange section is smallest of the tube number ratios, and the heat exchange section that is the lowermost heat exchange section and the auxiliary heat exchange section corresponding to the heat exchange section form the discharge accelerator. a plurality of flat tubes; a first header-collecting pipe connected to an end of each of the flat tubes; a second header-collecting pipe connected to the other end of each of the flat tubes; and a plurality of fins joined to the flat tubes, the heat exchanger provided in a refrigerant circuit which is configured to perform a refrigerating cycle, and causing a refrigerant to exchange heat with air, wherein the first header-collecting pipe and the second header-collecting pipe are in an upright position, a plurality of heat exchange sections (51a-51c) each of which is constituted by adjacent flat tubes arranged one above the other, the first header-collecting pipe includes therein one communicating space which communicates with the flat tubes of all of the heat exchange sections (51a-51c), the second header-collecting pipe includes therein subspaces (71a-71c) which correspond to the heat exchange sections (51a-51c) on a one-by-one basis and each communicate with the flat tubes constituting a corresponding one of the heat exchange sections (51a-51c), auxiliary heat exchange sections (52a-52c) constituted by flat tubes, which auxiliary heat exchange sections correspond to the heat exchange sections (51a-51c) on a one-by-one basis, the flat tubes constituting the auxiliary heat exchange sections (52a-52c) are smaller in number than the flat tubes constituting the heat exchange sections (51a-51c), and the auxiliary heat exchange sections (52a-52c) are each in series connection to a corresponding one of the heat exchange sections (51a-51c), characterized in that the heat exchanger further includes a discharge accelerator which accelerates discharge of the refrigerant in a liquid state from a lower portion of the heat exchange section which is the lowermost heat exchange section during defrosting in which the refrigerant in a high-pressure gas state is introduced from the communicating space to the flat tubes in order to melt frost having formed on the fins, tube number ratios are obtained by dividing the number of the flat tubes constituting each of the heat exchange sections (51a -51c) by the number of the flat tubes constituting a corresponding one of the auxiliary heat exchange sections (52a-52c), the tube number ratio of the heat exchange section that is the lowermost heat exchange section is smallest of the tube number ratios, and the heat exchange section that is the lowermost heat exchange section and the auxiliary heat exchange section corresponding to the heat exchange section form the discharge accelerator.