A running mower includes a traveling body 1 supported by a pair of front wheels 2 and a pair of rear wheels 3, a rotary mowing unit 12 vertically movably mounted to the traveling body between the front wheels and the rear wheels, and an outlet duct 23 extending backward for discharging mowed grass. A downwardly open channel-shaped upper duct 57 covers an outlet 58 of the rotary mowing unit for mowed grass. The upper duct has a rear end inserted into an upper front end of the outlet duct 23. The outlet duct 23 is channel-shaped to be downwardly open. The underside of the outlet duct is provided with an upwardly open channel-shaped lower duct 61 forming a bottom element of the outlet duct and having a vertically movable front end. The front end of the lower duct moves together with the rotary mowing unit, so that the mowed grass is smoothly discharged even if the mowing height of the rotary mowing unit is lowered.