A heat exchanger includes tubes, a tank, and a prevention plate arranged inside the tank and contacting end surfaces of the tubes to prevent heat medium from flowing into the tubes in a certain region. Flow paths are formed in each tube and arranged in a width direction of the tube. The prevention plate includes a deformable portion elastically deformable by contacting an edge of the opening at a time of inserting the prevention plate into the tank from the opening. The deformable portion of the prevention plate that has been inserted into the tank is restored and receives a reaction force from an inner surface of the tank such that the prevention plate is shifted along the width direction by the reaction force.