A mid-mount riding grass mower includes a belt transmission device attached to an upper portion of a mower unit, and a cover unit that covers the belt transmission device. The cover unit includes a lower cover extending upward from the mower deck so as to surround three sides of the belt transmission device, and right and left side covers detachably attached to right and left end portions of the traveling vehicle body. The lower cover has an upper end disposed higher than an upper end of the belt transmission device and has a vertical length for forming a maintenance space relative to a front/rear intermediate lower portion of the traveling vehicle body when the mower unit is located at its lower limit position.
