A heat exchanger comprising: a first flat tube and a second flat tube that extend in a direction crossing a flowing direction of a fluid, and that are arranged side by side in the flowing direction, refrigerant flowing through the first flat tube and the second flat tube; and a header configured to connect one end of the first flat tube and one end of the second flat tube, the header being formed of a first member to which the one end of the first flat tube and the one end of the second flat tube are fixed, and a second member connected so as to overlap with the first member (1 1a), the second member being provided with a recessed portion extending from a position that the one end of the first flat tube faces to a position that the one end of the second flat tube faces, wherein the first member is provided with a first opening and a second opening, the one end of the first flat tube is inserted into the first opening, the one end of the second flat tube is inserted into the second opening, wherein the first opening is located at a leading end of a first side wall portion in the first member protruding from the second member toward the first flat tube, the second opening is located at a leading end of a second side wall portion in the first member protruding from the second member toward the second flat tube, the heat exchanger being characterized in that the one end of the first flat tube that is inserted into the first opening is narrower in width than a center portion of the first flat tube in an extending direction thereof,the one end of the second flat tube that is inserted into the second opening is narrower in width than a center portion of the second flat tube in an extending direction thereof, and the header comprises a crimping portion configured to crimp the first member and the second member so as to be fixed, and the header further comprises a brazing material for connecting the one end of the first flat tube and the one end of the second flat tube to a surface of the first member, and in that the leading end of the first side wall portion is connected via the brazing material to a position that is distant from an end face of the one end of the first flat tube, and in that the leading end of the second side wall portion is connected via the brazing material (1 1b) to a position that is distant from an end face of the one end of the second flat tube, a width changing portion of the first flat tube is located at a leading end of a first side wall portion, and a brazing material is provided on the first member (1 1a) along a surface of the first member. a first flat tube and a second flat tube that extend in a direction crossing a flowing direction of a fluid, and that are arranged side by side in the flowing direction, refrigerant flowing through the first flat tube and the second flat tube; and a header configured to connect one end of the first flat tube and one end of the second flat tube, the header being formed of a first member to which the one end of the first flat tube and the one end of the second flat tube are fixed, and a second member connected so as to overlap with the first member (1 1a), the second member being provided with a recessed portion extending from a position that the one end of the first flat tube faces to a position that the one end of the second flat tube faces, wherein a first member to which the one end of the first flat tube and the one end of the second flat tube are fixed, and a second member connected so as to overlap with the first member (1 1a), the second member being provided with a recessed portion extending from a position that the one end of the first flat tube faces to a position that the one end of the second flat tube faces, wherein the first member is provided with a first opening and a second opening, the one end of the first flat tube is inserted into the first opening, the one end of the second flat tube is inserted into the second opening, wherein the first opening is located at a leading end of a first side wall portion in the first member protruding from the second member toward the first flat tube, the second opening is located at a leading end of a second side wall portion in the first member protruding from the second member toward the second flat tube, the heat exchanger being characterized in that the one end of the first flat tube that is inserted into the first opening is narrower in width than a center portion of the first flat tube in an extending direction thereof,the one end of the second flat tube that is inserted into the second opening is narrower in width than a center portion of the second flat tube in an extending direction thereof, and the header comprises a crimping portion configured to crimp the first member and the second member so as to be fixed, and the header further comprises a brazing material for connecting the one end of the first flat tube and the one end of the second flat tube to a surface of the first member, and in that the leading end of the first side wall portion is connected via the brazing material to a position that is distant from an end face of the one end of the first flat tube, and in that the leading end of the second side wall portion is connected via the brazing material (1 1b) to a position that is distant from an end face of the one end of the second flat tube, a width changing portion of the first flat tube is located at a leading end of a first side wall portion, and a brazing material is provided on the first member (1 1a) along a surface of the first member.