A volute pump comprising: an impeller rotatable together with a rotational shaft; and an impeller casing having a suction port and a volute chamber; wherein a groove, extending from the suction port to the volute chamber, is formed in an inner surface of the impeller casing, wherein the impeller includes a hub to which the rotational shaft is fixed, and a sweep-back vane extending helically from the hub in a direction opposite to a rotating direction of the impeller, wherein the groove has an inlet which is connected to the suction port, wherein the sweep-back vane includes a leading edge portion extending helically from the hub and having an outer end, and a trailing edge portion extending helically from the leading edge portion, wherein, when the impeller is rotated, the outer end of the leading edge portion moves across an inlet of the groove, and wherein the groove is located so as to face the trailing edge portion of the sweep-back vane, and characterized in that the leading edge portion has a front-side curved surface extending from an inner end to the outer end of the leading edge portion, the front-side curved surface being a surface of the leading edge portion which is located at a foremost position in the rotating direction of the impeller, and a cross-section of the front-side curved surface in a thickness direction of the sweep-back vane has an arc shape with a radius of curvature. an impeller rotatable together with a rotational shaft; and an impeller casing having a suction port and a volute chamber; wherein a groove, extending from the suction port to the volute chamber, is formed in an inner surface of the impeller casing, wherein the impeller includes a hub to which the rotational shaft is fixed, and a sweep-back vane extending helically from the hub in a direction opposite to a rotating direction of the impeller, wherein the groove has an inlet which is connected to the suction port, wherein the sweep-back vane includes a leading edge portion extending helically from the hub and having an outer end, and a trailing edge portion extending helically from the leading edge portion, wherein, when the impeller is rotated, the outer end of the leading edge portion moves across an inlet of the groove, and wherein the groove is located so as to face the trailing edge portion of the sweep-back vane, wherein the impeller includes a hub to which the rotational shaft is fixed, and a sweep-back vane extending helically from the hub in a direction opposite to a rotating direction of the impeller, wherein the groove has an inlet which is connected to the suction port, a hub to which the rotational shaft is fixed, and a sweep-back vane extending helically from the hub in a direction opposite to a rotating direction of the impeller, wherein the groove has an inlet which is connected to the suction port, wherein the sweep-back vane includes a leading edge portion extending helically from the hub and having an outer end, and a trailing edge portion extending helically from the leading edge portion, a leading edge portion extending helically from the hub and having an outer end, and a trailing edge portion extending helically from the leading edge portion, wherein, when the impeller is rotated, the outer end of the leading edge portion moves across an inlet of the groove, and wherein the groove is located so as to face the trailing edge portion of the sweep-back vane,