Disclosed is a self-reciprocating energy recovery device utilized in driving of a seawater pump by self-reciprocating a piston of a power recovery chamber and recovering energy not using an electronic drive unit but using the hydraulic power of concentrated water. The self-reciprocating energy recovery device including a pair of power recovery chambers having pistons therein respectively, a high-pressure concentrated supply pipe, a low-pressure concentrated discharge pipe, and a high-pressure seawater discharge pipe to enable the power recovery chambers to recover hydraulic power supplied through the high-pressure concentrated water supply pipe and utilize the hydraulic power in driving of a seawater pump.