<p>PURPOSE: A header unit and a heat exchanger are provided to prevent a body and a cover from being not welded inside a header and to prevent a leakage of a refrigerant to a space between both tanks inside the header. CONSTITUTION: A header unit includes a body (24) and a cover (21) joined to the body. The body includes a base part (26) and a middle partition (27) protruded on the base part. The cover includes a joining groove so that the middle partition is joined to the groove by penetrating through the joining groove. The middle partition includes a first middle partition (27) extended from the base part and a second middle partition (28) extended from the first middle partition.</p>
