A mower unit includes a mower deck mounting therein in parallel layout first, second and third rotary blades rotatably driven about vertical axes and a washing device. The mower deck includes a top wall, side walls and first, second and third auxiliary side walls for evacuation. The washing device includes a single water supplying unit disposed upwardly of the top wall for supplying washing water and first, second and third water discharging units disposed through the first, second and third auxiliary side walls for discharging water to the first, second and third rotary blades respectively. The washing device includes also a water supplying pipe unit interconnecting and communicating the water supplying unit to the first, second and third water discharging units. The water supplying pipe unit is disposed within a mower deck rear space delimited by outer faces of the first, second and third auxiliary side walls.