A mower unit is detachably mounted between front wheels and rear wheels of a mid-mount grass mower. The mower unit includes a blade cover having a top plate and a side plate extending from a circumferential edge of the top plate, a pair of right and left ride-over deck units provided on the top plate. Each ride-over deck unit includes a support deck fixed to an upper face of the top plate, a wheel guide member movable between a home position for storage in the support deck and a bridging position for bridging between the support deck and a ground surface, and a tethering member for tethering the support deck with the wheel guide member.
