To recover pressure energy by using as a medium, fluid not including combustion products such as particulate matters.

This energy recovery device has an air compression unit 62 arranged between an exhaust passage 28 and a pressure accumulation container 60 in an internal combustion engine 10 and including a first partitioned chamber R1 capable of communicating with the exhaust passage 28, a second partitioned chamber R2 capable of communicating with the pressure accumulation container 60, and a partitioning member 66 for separating the first partitioned chamber R1 from the second partitioned chamber R2. The partitioning member 66 can be moved between a sucked condition where air is sucked into the second partitioned chamber R2 and a compressively supplied condition where fluid in the exhaust passage 28 is taken into the first partitioned chamber R2 and air is compressively supplied to the pressure accumulation container 60, based on a pressure difference between the first partitioned chamber R1 and the second partitioned chamber R2. Therefore, pressure energy of fluid taken into the first partitioned chamber R1 of the air compression unit 62 is transmitted to air in the second partitioned chamber R2 through the partitioning member 66 and recovered to the pressure accumulation container 60.