A grass mower device comprising: a cutter blade housing; a cutter blade mounted inside the cutter blade housing to be rotatable about a vertically oriented axis, a discharge opening being formed at a lateral outer side of the cutter blade housing for discharging grass clippings produced by the cutter blade to the outside of the cutter blade housing; and a discharge-opening cover pivotable between a lowered closing posture for covering an upper side of the discharge opening and an elevated opening posture for uncovering the discharge opening; a cover urging mechanism connected to the cutter blade housing and the discharge-opening cover for urging the discharge-opening cover to the closing posture; a lock mechanism switchable between a locking state for holding the discharge-opening cover under the closing posture and a lock releasing posture for allowing switchover of the discharge-opening cover to the opening posture; a lock urging mechanism connected to the cutter blade housing and the lock mechanism for urging the lock mechanism to the locking state, the lock urging mechanism being configured to apply an urging force to the lock mechanism in response to switchover of the discharge-opening cover to the closing posture; and an operational portion for switching the lock mechanism to the lock releasing posture against the urging force of the lock urging mechanism; characterized in that the lock mechanism comprises a stopper member pivotally operated about a vertically-oriented support shaft of the cutter blade housing; and the stopper member is displaceable between a stopper position for coming into contact with the discharge-opening cover to inhibit elevation of the discharge-opening cover from the closing posture, and a stopper releasing position for releasing the contact. a cutter blade housing; a cutter blade mounted inside the cutter blade housing to be rotatable about a vertically oriented axis, a discharge opening being formed at a lateral outer side of the cutter blade housing for discharging grass clippings produced by the cutter blade to the outside of the cutter blade housing; and a discharge-opening cover pivotable between a lowered closing posture for covering an upper side of the discharge opening and an elevated opening posture for uncovering the discharge opening; a cover urging mechanism connected to the cutter blade housing and the discharge-opening cover for urging the discharge-opening cover to the closing posture; a lock mechanism switchable between a locking state for holding the discharge-opening cover under the closing posture and a lock releasing posture for allowing switchover of the discharge-opening cover to the opening posture; a lock urging mechanism connected to the cutter blade housing and the lock mechanism for urging the lock mechanism to the locking state, the lock urging mechanism being configured to apply an urging force to the lock mechanism in response to switchover of the discharge-opening cover to the closing posture; and an operational portion for switching the lock mechanism to the lock releasing posture against the urging force of the lock urging mechanism; characterized in that the lock mechanism comprises a stopper member pivotally operated about a vertically-oriented support shaft of the cutter blade housing; and the stopper member is displaceable between a stopper position for coming into contact with the discharge-opening cover to inhibit elevation of the discharge-opening cover from the closing posture, and a stopper releasing position for releasing the contact.