An air conditioner comprising: a plurality of refrigerant tubes provided in parallel; and a header configured to introduce a refrigerant into the plurality of refrigerant tubes, the header comprising: a main header chamber communicating with the plurality of refrigerant tubes; a refrigerant inlet pipe horizontally provided in the main header chamber and configured to introduce the refrigerant horizontally into the main header chamber; a flow direction changing mechanism is provided in the main header chamber, and is configured to change a flow of the refrigerant from a horizontal direction to an upward direction as the refrigerant introduced from the refrigerant inlet pipe collides with the flow direction changing mechanism, wherein when the refrigerant flows to the upper side of the main header chamber, the refrigerant flows into the plurality of refrigerant tubes; and a plurality of sub header chambers branched substantially perpendicular to the main header chamber and provided in parallel, wherein each of the plurality of sub header chambers is respectively connected to each of the plurality of refrigerant tubes, and wherein the refrigerant introduced into the main header chamber is distributed into the plurality of refrigerant tubes through the plurality of sub header chambers; and characterised in that: the main header chamber and the plurality of sub header chambers are provided inside a header tube, the main header chamber is formed by an inner surface of the header tube and a first plate member provided to partition the inside of the header tube, and the plurality of sub header chambers are formed by the inner surface of the header tube, the first plate member, and a plurality of second plate members provided to partition the inside of the header tube. a plurality of refrigerant tubes provided in parallel; and a header configured to introduce a refrigerant into the plurality of refrigerant tubes, the header comprising: a main header chamber communicating with the plurality of refrigerant tubes; a refrigerant inlet pipe horizontally provided in the main header chamber and configured to introduce the refrigerant horizontally into the main header chamber; a flow direction changing mechanism is provided in the main header chamber, and is configured to change a flow of the refrigerant from a horizontal direction to an upward direction as the refrigerant introduced from the refrigerant inlet pipe collides with the flow direction changing mechanism, wherein when the refrigerant flows to the upper side of the main header chamber, the refrigerant flows into the plurality of refrigerant tubes; and a plurality of sub header chambers branched substantially perpendicular to the main header chamber and provided in parallel, a main header chamber communicating with the plurality of refrigerant tubes; a refrigerant inlet pipe horizontally provided in the main header chamber and configured to introduce the refrigerant horizontally into the main header chamber; a flow direction changing mechanism is provided in the main header chamber, and is configured to change a flow of the refrigerant from a horizontal direction to an upward direction as the refrigerant introduced from the refrigerant inlet pipe collides with the flow direction changing mechanism, wherein when the refrigerant flows to the upper side of the main header chamber, the refrigerant flows into the plurality of refrigerant tubes; and a plurality of sub header chambers branched substantially perpendicular to the main header chamber and provided in parallel, wherein each of the plurality of sub header chambers is respectively connected to each of the plurality of refrigerant tubes, and wherein the refrigerant introduced into the main header chamber is distributed into the plurality of refrigerant tubes through the plurality of sub header chambers; and characterised in that: the main header chamber and the plurality of sub header chambers are provided inside a header tube, the main header chamber is formed by an inner surface of the header tube and a first plate member provided to partition the inside of the header tube, and the plurality of sub header chambers are formed by the inner surface of the header tube, the first plate member, and a plurality of second plate members provided to partition the inside of the header tube. the main header chamber and the plurality of sub header chambers are provided inside a header tube, the main header chamber is formed by an inner surface of the header tube and a first plate member provided to partition the inside of the header tube, and the plurality of sub header chambers are formed by the inner surface of the header tube, the first plate member, and a plurality of second plate members provided to partition the inside of the header tube.