A grass mower device includes a cover urging mechanism (26) for urging a discharge opening (20) to a closing posture, a lock mechanism (30) switchable between a locking state for holding the discharge opening (20) cover under the closing posture and a lock releasing posture for allowing switchover of the discharge-opening cover (20) to the opening posture, a lock urging mechanism (34) for urging the lock mechanism (30) to the locking state, and an operational portion (33) for switching the lock mechanism (34) to the lock releasing posture against the urging force of the lock urging mechanism (34).