To provide a heat exchanger for performing heat exchange by boiling refrigerant and evaporating a part of the refrigerant and capable of realizing excellent heat exchange efficiency.

A refrigerant flow passage 3 in which the refrigerant flows in a perpendicular direction to an input direction of heat transferred from a heat source 4 is provided and at least a part of the refrigerant is evaporated in the refrigerant flow passage 3 by heat exchange. The refrigerant flow passage 3 is equipped with a main passage 13 and a recessed part 14 which is formed at a heat source 4 side of the main passage 13 along its longitudinal direction and is narrower than the main passage 13. The recessed part 14 is formed so that air bubble generated by evaporation can be discharged to the main passage 13 when at least a part of the refrigerant is evaporated in the recessed part 14 by heat exchange. Width w1 of the recessed part 14 is 0.1 to 3 mm, favorably, 0.1 to 1 mm. When the width w1 of the recessed part 14 is 0.1 to 1 mm, depth d1 of the recessed part 14 is 1/5 to 1/2 of the width w1.