A riding lawn mower comprising: a housing opening downward; a cut grass receptacle disposed rearward of the housing; left and right cutting blades (31 L, 31 R) accommodated in the housing and rotatable in generally a same plane in such a manner as to not only cut grass but also generate cut grass delivering air; a cut grass delivery passage for delivering grass, cut by the left and right cutting blades (31 L, 31 R), from a widthwise center portion of the housing to the cut grass receptacle by use of the cut grass delivering air; a mechanism for switching the lawn mower between a bagging mode in which grass cut by the cutting blades (31 L, 31 R) is delivered from the housing to the cut grass receptacle and a mulching mode in which grass cut by the cutting blades is discharged downward of the housing; and a scroll guide provided in an entrance of the cut grass delivery passage for directing the cut grass to the cut grass delivery passage and the delivering air while swirling the cut grass and the delivering air within the housing, wherein an overall shape of the scroll guide is, as viewed in top plan, a substantial isosceles triangle extending into the housing while tapering forward from a base section of the scroll guide located in the entrance, and, as viewed in side elevation, a substantial triangle tapering forward from the base section, characterized in that left and right side surfaces (73L, 73R) of the scroll guide each comprise a lower slanting surface and an upper slanting surface formed as different slants below and above a substantial middle height portion thereof, each of the lower slanting surface and the upper slanting surface being shaped to gradually increase in area in a direction from a distal end of the scroll guide toward the base section. a housing opening downward; a cut grass receptacle disposed rearward of the housing; left and right cutting blades (31 L, 31 R) accommodated in the housing and rotatable in generally a same plane in such a manner as to not only cut grass but also generate cut grass delivering air; a cut grass delivery passage for delivering grass, cut by the left and right cutting blades (31 L, 31 R), from a widthwise center portion of the housing to the cut grass receptacle by use of the cut grass delivering air; a mechanism for switching the lawn mower between a bagging mode in which grass cut by the cutting blades (31 L, 31 R) is delivered from the housing to the cut grass receptacle and a mulching mode in which grass cut by the cutting blades is discharged downward of the housing; and a scroll guide provided in an entrance of the cut grass delivery passage for directing the cut grass to the cut grass delivery passage and the delivering air while swirling the cut grass and the delivering air within the housing, wherein an overall shape of the scroll guide is, as viewed in top plan, a substantial isosceles triangle extending into the housing while tapering forward from a base section of the scroll guide located in the entrance, and, as viewed in side elevation, a substantial triangle tapering forward from the base section, characterized in that left and right side surfaces (73L, 73R) of the scroll guide each comprise a lower slanting surface and an upper slanting surface formed as different slants below and above a substantial middle height portion thereof, each of the lower slanting surface and the upper slanting surface being shaped to gradually increase in area in a direction from a distal end of the scroll guide toward the base section.