A cross flow fan comprising: impeller units including disk-shaped support plates whose center of rotation is located in a center thereof and a plurality of blades extending in a rotation-axis direction and arranged along outer peripheries of the support plates, the blades each being supported by the support plates at opposite ends; an impeller formed by fixing the impeller units in the rotation-axis direction; a plurality of grooves having a depressed shape that extend in the rotation-axis direction, the grooves being provided on a blade suction surface serving as a back side in a rotational direction of each blade; and a flat section is provided between neighboring grooves by a predetermined distance, wherein the distance between the neighboring grooves is larger than a groove depth (h) of the grooves, each of the grooves is constituted by a groove bottom and groove sides that face each other, the groove bottom and connection areas between the groove sides and the blade suction surface have a rounded shape, wherein in a cross section taken in a direction perpendicular to a rotation axis, the blades each have a substantially circular-arc shape between a leading edge located on an outer circumference side of the support plates and a trailing edge located on an inner circumference side of the support plates, a blade thickness of a midsection between the leading edge and the trailing edge is larger than blade thicknesses at the leading edge and the trailing edge, and wherein the grooves provided on the blade suction surface are disposed so as to have at least a distance equivalent to the blade thickness of the leading edge or the trailing edge from a blade pressure surface serving as a leading surface in the rotational direction of each blade, and the grooves are provided near at least the leading edge, the trailing edge and the midsection, characterized in that at least a groove width (g) or a groove depth (h) of each groove increases in the rotation-axis direction and the groove depth (h) on the leading edge side and the trailing edge side is made smaller than the groove depth (h) in a midsection that is between the leading edge and the trailing edge. impeller units including disk-shaped support plates whose center of rotation is located in a center thereof and a plurality of blades extending in a rotation-axis direction and arranged along outer peripheries of the support plates, the blades each being supported by the support plates at opposite ends; an impeller formed by fixing the impeller units in the rotation-axis direction; a plurality of grooves having a depressed shape that extend in the rotation-axis direction, the grooves being provided on a blade suction surface serving as a back side in a rotational direction of each blade; and a flat section is provided between neighboring grooves by a predetermined distance, wherein the distance between the neighboring grooves is larger than a groove depth (h) of the grooves, each of the grooves is constituted by a groove bottom and groove sides that face each other, the groove bottom and connection areas between the groove sides and the blade suction surface have a rounded shape, wherein in a cross section taken in a direction perpendicular to a rotation axis, the blades each have a substantially circular-arc shape between a leading edge located on an outer circumference side of the support plates and a trailing edge located on an inner circumference side of the support plates, a blade thickness of a midsection between the leading edge and the trailing edge is larger than blade thicknesses at the leading edge and the trailing edge, and wherein the grooves provided on the blade suction surface are disposed so as to have at least a distance equivalent to the blade thickness of the leading edge or the trailing edge from a blade pressure surface serving as a leading surface in the rotational direction of each blade, and the grooves are provided near at least the leading edge, the trailing edge and the midsection, characterized in that at least a groove width (g) or a groove depth (h) of each groove increases in the rotation-axis direction and the groove depth (h) on the leading edge side and the trailing edge side is made smaller than the groove depth (h) in a midsection that is between the leading edge and the trailing edge.