A grass mowing machine comprising: a mower disposed under a machine body and configured to carry out a grass cutting work; a grass collecting section disposed at a rear portion of the machine body and configured to collect cut grass pieces; and a duct for guiding the cut grass pieces cut by the mower to the grass collecting section; wherein the mower includes: a discharging section formed cylindrical and configured to discharge the cut grass pieces to the duct; and a guide member which forms a bottom portion of the discharging section and which is vertically pivotable; wherein the duct includes: a duct body; and a bottom plate provided at a lower portion of the inner space of the duct body; and a guide operational lever manually operable for pivotally operating the guide member; wherein the bottom plate is supported to be vertically pivotable between a cut grass conveying posture in which a front portion thereof is placed close to a rear portion of the guide member and a maintenance posture in which the front portion is placed upwardly away from the rear portion of the guide member; and characterized in that a bottom plate operational tool is provided for switching over between a first operational state for switching the posture of the bottom plate to the cut grass conveying posture and maintaining the bottom plate under the cut grass conveying posture and a second operational state for switching the posture of the bottom plate to the maintenance posture and maintaining the bottom plate under the maintenance posture. a mower disposed under a machine body and configured to carry out a grass cutting work; a grass collecting section disposed at a rear portion of the machine body and configured to collect cut grass pieces; and a duct for guiding the cut grass pieces cut by the mower to the grass collecting section; wherein the mower includes: a discharging section formed cylindrical and configured to discharge the cut grass pieces to the duct; and a guide member which forms a bottom portion of the discharging section and which is vertically pivotable; a discharging section formed cylindrical and configured to discharge the cut grass pieces to the duct; and a guide member which forms a bottom portion of the discharging section and which is vertically pivotable; wherein the duct includes: a duct body; and a bottom plate provided at a lower portion of the inner space of the duct body; and a duct body; and a bottom plate provided at a lower portion of the inner space of the duct body; and a guide operational lever manually operable for pivotally operating the guide member; wherein the bottom plate is supported to be vertically pivotable between a cut grass conveying posture in which a front portion thereof is placed close to a rear portion of the guide member and a maintenance posture in which the front portion is placed upwardly away from the rear portion of the guide member; and characterized in that a bottom plate operational tool is provided for switching over between a first operational state for switching the posture of the bottom plate to the cut grass conveying posture and maintaining the bottom plate under the cut grass conveying posture and a second operational state for switching the posture of the bottom plate to the maintenance posture and maintaining the bottom plate under the maintenance posture.