A riding lawn mower comprising: a housing opening downward and having a cutting blade (31L, 31R) accommodated therein; a cut grass receptacle disposed rearward of the housing; a cut grass delivery passage for delivering grass, cut by the cutting blade (31L, 31R), from the housing to the cut grass receptacle; and a shutter mechanism provided on a lower portion of the cut grass delivery passage for switching the lawn mower between a bagging mode in which grass cut by the cutting blade is delivered from the housing to the cut grass receptacle and a mulching mode in which grass cut by the cutting blade is discharged downward of the housing, the shutter mechanism including a horizontal pivot shaft disposed on the lower portion of the cut grass delivery passage, a shutter plate pivotable about the pivot shaft between a fully closed position where the shutter plate closes the cut grass delivery passage in order to execute the mulching mode and a fully opened position in order to execute the bagging mode, and an operating force transmission section for transmitting opening/closing operation force, given via an operation section being disposed away from the shutter mechanism, to the shutter plate, wherein the operation section is operable to switch the shutter plate between the fully closed position and the fully opened position, characterized in that in the fully opened position of the cut grass delivery passage, the shutter plate is laid down toward a discharge side of the cut grass delivery passage, and that the shutter mechanism is constructed as a single unit integrally including: a base mounted on a lower portion of the cut grass delivery passage, the pivot shaft and the shutter plate assembled to the base, and the operating force transmission section, wherein this single unit is detachably attached to the cut grass delivery passage. a housing opening downward and having a cutting blade (31L, 31R) accommodated therein; a cut grass receptacle disposed rearward of the housing; a cut grass delivery passage for delivering grass, cut by the cutting blade (31L, 31R), from the housing to the cut grass receptacle; and a shutter mechanism provided on a lower portion of the cut grass delivery passage for switching the lawn mower between a bagging mode in which grass cut by the cutting blade is delivered from the housing to the cut grass receptacle and a mulching mode in which grass cut by the cutting blade is discharged downward of the housing, the shutter mechanism including a horizontal pivot shaft disposed on the lower portion of the cut grass delivery passage, a shutter plate pivotable about the pivot shaft between a fully closed position where the shutter plate closes the cut grass delivery passage in order to execute the mulching mode and a fully opened position in order to execute the bagging mode, and an operating force transmission section for transmitting opening/closing operation force, given via an operation section being disposed away from the shutter mechanism, to the shutter plate, wherein the operation section is operable to switch the shutter plate between the fully closed position and the fully opened position, characterized in that in the fully opened position of the cut grass delivery passage, the shutter plate is laid down toward a discharge side of the cut grass delivery passage, and that the shutter mechanism is constructed as a single unit integrally including: a base mounted on a lower portion of the cut grass delivery passage, the pivot shaft and the shutter plate assembled to the base, and the operating force transmission section, wherein this single unit is detachably attached to the cut grass delivery passage.