A waste heat recovery device (100;100a;100b;100c;100d;100e;100j;100k) comprising: a low-boiling-point medium Rankine cycle (150;150e;150j;150k) arranged such that a low-boiling-point medium circulates therein while the low-boiling-point medium is repeatedly condensed and evaporated; a heated water line (171;171b;171g;171m) that is arranged to guide heated liquid water to the low-boiling-point medium Rankine cycle (150;150e;150j;150k) from a waste heat recovery boiler (110;110j) arranged to heat water by exhaust gas generated from a gas turbine (10;10a;10b;10j); and a water recovery line (178;178d;178g) that is arranged to return the water, which has been led from the heated water line (171;171b;171g;171m) to the low-boiling-point medium Rankine cycle (150;150e;150j;150k), and passed through the low-boiling-point medium Rankine cycle (150;150e;150j;150k), to the waste heat recovery boiler (110;110j), wherein the low-boiling-point medium Rankine cycle (150;150e;150j;150k) includes a heater (151;155;155c) that is arranged to heat the low-boiling-point medium by exchanging heat between the low-boiling-point medium, which is a liquid, and water, which is the liquid having passed through the heated water line (171;171b;171g;171m), wherein the waste heat recovery boiler (110;110j) includes at least one or more evaporators (113a-c) that are arranged to generate steam by heating water by the exhaust gas, and economizers (112a-d) that are provided for the respective one or more evaporators (113a-c) and are arranged to heat water, which is sent to the evaporators (113a-c), by the exhaust gas, wherein the heated water line (171;171b;171g;171m) is arranged to guide liquid water, which is heated by either of the economizers (112a-d) provided for the respective one or more evaporators (113a-c), to the low-boiling-point medium Rankine cycle (150;150e;150j;150k), and wherein the water recovery line (178;178d;178g) is arranged to return water, which has passed through the low-boiling-point medium Rankine cycle (150;150e;150j;150k), to the waste heat recovery boiler (110;110j) through a water supply line (131;131a,149) that is arranged to supply water to the waste heat recovery boiler (110;110j). a low-boiling-point medium Rankine cycle (150;150e;150j;150k) arranged such that a low-boiling-point medium circulates therein while the low-boiling-point medium is repeatedly condensed and evaporated; a heated water line (171;171b;171g;171m) that is arranged to guide heated liquid water to the low-boiling-point medium Rankine cycle (150;150e;150j;150k) from a waste heat recovery boiler (110;110j) arranged to heat water by exhaust gas generated from a gas turbine (10;10a;10b;10j); and a water recovery line (178;178d;178g) that is arranged to return the water, which has been led from the heated water line (171;171b;171g;171m) to the low-boiling-point medium Rankine cycle (150;150e;150j;150k), and passed through the low-boiling-point medium Rankine cycle (150;150e;150j;150k), to the waste heat recovery boiler (110;110j), wherein the low-boiling-point medium Rankine cycle (150;150e;150j;150k) includes a heater (151;155;155c) that is arranged to heat the low-boiling-point medium by exchanging heat between the low-boiling-point medium, which is a liquid, and water, which is the liquid having passed through the heated water line (171;171b;171g;171m), wherein the waste heat recovery boiler (110;110j) includes at least one or more evaporators (113a-c) that are arranged to generate steam by heating water by the exhaust gas, and economizers (112a-d) that are provided for the respective one or more evaporators (113a-c) and are arranged to heat water, which is sent to the evaporators (113a-c), by the exhaust gas, wherein the heated water line (171;171b;171g;171m) is arranged to guide liquid water, which is heated by either of the economizers (112a-d) provided for the respective one or more evaporators (113a-c), to the low-boiling-point medium Rankine cycle (150;150e;150j;150k), and wherein the water recovery line (178;178d;178g) is arranged to return water, which has passed through the low-boiling-point medium Rankine cycle (150;150e;150j;150k), to the waste heat recovery boiler (110;110j) through a water supply line (131;131a,149) that is arranged to supply water to the waste heat recovery boiler (110;110j).