A dust removal system for a riding lawn mower, comprising an engine, a speed change device disposed outside of an engine compartment, and a universal coupling inserted through an opening provided to a separating wall in order to connect an output shaft of the engine and an input shaft of the speed change device. The dust removal system is composed of a perforated board which rotates integrally with the output shaft and around the axis of the output shaft in order to impede grass cuttings from flowing into the engine compartment through the opening, the perforated board having an insertion hole formed in the center to allow the universal coupling to be inserted; and a seal unit for substantially sealing the space between the universal coupling and the peripheral edge of the insertion hole. The seal unit has a radially extending flange formed on the universal coupling, and a ring member in which one end is mounted on the flange and the other end is in contact with the peripheral area around the insertion hole.