A heat exchanger is for use in an energy recovery system to be mounted on a vessel including an engine, a supercharger and an economizer, the heat exchanger including: a first heat section for heating a working medium by supercharged air from the supercharger; a second heat section for heating the supercharged air by steam generated by the economizer before the supercharged air flows into the first heat section; and a third heat section for heating the working medium having been heated in the first section by the supercharged air which is to be heated in the second section.