A dissolved air flotation device, comprising: a raw water supply means (2, 21) that adjusts a flow rate of raw water supplied; a separation tank having a circular cross section and having a raw water inlet which is opened in a direction of tangent with respect to the body of the separation tank and to which a raw water inflow pipe is connected, wherein a connection end of the raw water inflow pipe is located at a height which is 5 to 60% of the water level in the separation tank so that an ascending swirling flow can be easily formed, such that raw water supplied by the raw water supply means (2, 21) swirls and flows in a predetermined direction within the separation tank while the raw water is introduced into the separation tank through the raw water inflow pipe, a treated water outlet to which a treated water outflow pipe that vertically extends and has a circular cross section is connected, wherein an upper end of the treated water outflow pipe to which treated water is introduced from the separation tank is positioned at a height which is 5 to 50% of the water level in the separation tank, and a sediment discharge outlet through which sediments on a bottom of the separation tank are discharged outside; the separation tank further comprises an upper portion that has a cylinder shape, a middle portion that has a reversed conical shape tapered to a lower side, and a lower portion that has a cylinder shape smaller than the upper portion; and a sediment discharge pipe is connected to the sediment discharge outlet in the lower portion of the separation tank; and a scum outflow pipe that has a circular cross section and vertically extends through the separation tank while passing through a center of the treated water outflow pipe, wherein a space that serves as an outflow path is formed between the treated water outflow pipe and the scum outflow pipe; and the treated water outflow pipe and the scum outflow pipe pass through a center of the lower portion of the separation tank; and a bubble supplier that supplies air bubbles to the raw water flowing along the raw water supply pipe, such that a mixture of the raw water and air bubbles is introduced into the separation tank, wherein the raw water and air bubbles ascends while forming a swirling flow around the wall of the separation tank, wherein the device is configured in such a way that contaminants and air bubbles that can be floated are gathered at the surface of water and then removed after being floated; treated water forms a descending swirling at a center portion of the separation tank and is discharged through the outflow pipe, wherein the separation tank, the treated water outflow pipe and the scum outflow pipe all are concentrically arranged with respect to a center of the swirling flow formed in the separation tank. a raw water supply means (2, 21) that adjusts a flow rate of raw water supplied; a separation tank having a circular cross section and having a raw water inlet which is opened in a direction of tangent with respect to the body of the separation tank and to which a raw water inflow pipe is connected, wherein a connection end of the raw water inflow pipe is located at a height which is 5 to 60% of the water level in the separation tank so that an ascending swirling flow can be easily formed, such that raw water supplied by the raw water supply means (2, 21) swirls and flows in a predetermined direction within the separation tank while the raw water is introduced into the separation tank through the raw water inflow pipe, a treated water outlet to which a treated water outflow pipe that vertically extends and has a circular cross section is connected, wherein an upper end of the treated water outflow pipe to which treated water is introduced from the separation tank is positioned at a height which is 5 to 50% of the water level in the separation tank, and a sediment discharge outlet through which sediments on a bottom of the separation tank are discharged outside; the separation tank further comprises an upper portion that has a cylinder shape, a middle portion that has a reversed conical shape tapered to a lower side, and a lower portion that has a cylinder shape smaller than the upper portion; and a sediment discharge pipe is connected to the sediment discharge outlet in the lower portion of the separation tank; and a scum outflow pipe that has a circular cross section and vertically extends through the separation tank while passing through a center of the treated water outflow pipe, wherein a space that serves as an outflow path is formed between the treated water outflow pipe and the scum outflow pipe; and the treated water outflow pipe and the scum outflow pipe pass through a center of the lower portion of the separation tank; and a bubble supplier that supplies air bubbles to the raw water flowing along the raw water supply pipe, such that a mixture of the raw water and air bubbles is introduced into the separation tank, wherein the raw water and air bubbles ascends while forming a swirling flow around the wall of the separation tank, wherein the device is configured in such a way that contaminants and air bubbles that can be floated are gathered at the surface of water and then removed after being floated; treated water forms a descending swirling at a center portion of the separation tank and is discharged through the outflow pipe, wherein the separation tank, the treated water outflow pipe and the scum outflow pipe all are concentrically arranged with respect to a center of the swirling flow formed in the separation tank.