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 (r1). 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 , <EOS>