A heat exchanger comprising: a header plate including tube insertion holes disposed in parallel spaced from each other in a longitudinal direction and, to the tube insertion hole, an end part of a flat tube is inserted; and a resin tank body fixed to the header plate via an annular packing, wherein: the header plate includes a bottom part provided with a seal face with which the packing makes contact and an insertion hole drilling face for which the tube insertion hole is provided, and a peripheral wall extended upward from an outer peripheral edge of the bottom part with a caulking claw formed at an edge part of the bottom part; for the insertion hole drilling face of the bottom part, a convex part protruding in the extending direction of the peripheral wall is provided in a range not exceeding an opening of the flat tube; the tank body includes: a flange part provided for an opening of the tank body, and a number of tooth parts protruding from an end face of the flange part along an inner periphery of the flange part, being spaced from each other and facing the seal face of the header plate; a tube end release part formed between the tooth parts and recessed, wherein a side edge in a longer axis direction of an opening of the flat tube is disposed in the tube end release part; and a packing accommodation part formed among an outer side face of each tooth part and the end face of the flange part and an inner peripheral face of the peripheral wall; an inside tip edge of the tooth part is seated on an outer peripheral side edge of the convex part; and the packing is arranged between the packing accommodation part and the seal face, and the tank body is fixed to the header plate by the caulking claw; characterised in that a gap is present between the side face of the tooth part and the packing. a header plate including tube insertion holes disposed in parallel spaced from each other in a longitudinal direction and, to the tube insertion hole, an end part of a flat tube is inserted; and a resin tank body fixed to the header plate via an annular packing, wherein: the header plate includes a bottom part provided with a seal face with which the packing makes contact and an insertion hole drilling face for which the tube insertion hole is provided, and a peripheral wall extended upward from an outer peripheral edge of the bottom part with a caulking claw formed at an edge part of the bottom part; for the insertion hole drilling face of the bottom part, a convex part protruding in the extending direction of the peripheral wall is provided in a range not exceeding an opening of the flat tube; the tank body includes: a flange part provided for an opening of the tank body, and a number of tooth parts protruding from an end face of the flange part along an inner periphery of the flange part, being spaced from each other and facing the seal face of the header plate; a tube end release part formed between the tooth parts and recessed, wherein a side edge in a longer axis direction of an opening of the flat tube is disposed in the tube end release part; and a packing accommodation part formed among an outer side face of each tooth part and the end face of the flange part and an inner peripheral face of the peripheral wall; an inside tip edge of the tooth part is seated on an outer peripheral side edge of the convex part; and the packing is arranged between the packing accommodation part and the seal face, and the tank body is fixed to the header plate by the caulking claw; characterised in that a gap is present between the side face of the tooth part and the packing. the header plate includes a bottom part provided with a seal face with which the packing makes contact and an insertion hole drilling face for which the tube insertion hole is provided, and a peripheral wall extended upward from an outer peripheral edge of the bottom part with a caulking claw formed at an edge part of the bottom part; for the insertion hole drilling face of the bottom part, a convex part protruding in the extending direction of the peripheral wall is provided in a range not exceeding an opening of the flat tube; the tank body includes: a flange part provided for an opening of the tank body, and a number of tooth parts protruding from an end face of the flange part along an inner periphery of the flange part, being spaced from each other and facing the seal face of the header plate; a tube end release part formed between the tooth parts and recessed, wherein a side edge in a longer axis direction of an opening of the flat tube is disposed in the tube end release part; and a packing accommodation part formed among an outer side face of each tooth part and the end face of the flange part and an inner peripheral face of the peripheral wall; a flange part provided for an opening of the tank body, and a number of tooth parts protruding from an end face of the flange part along an inner periphery of the flange part, being spaced from each other and facing the seal face of the header plate; a tube end release part formed between the tooth parts and recessed, wherein a side edge in a longer axis direction of an opening of the flat tube is disposed in the tube end release part; and a packing accommodation part formed among an outer side face of each tooth part and the end face of the flange part and an inner peripheral face of the peripheral wall; an inside tip edge of the tooth part is seated on an outer peripheral side edge of the convex part; and the packing is arranged between the packing accommodation part and the seal face, and the tank body is fixed to the header plate by the caulking claw;