A mid-mount riding grass mower comprising: a traveling vehicle body including a vehicle body frame which includes right and left side members; a mower unit liftably suspension-supported at a lower portion of the traveling vehicle body that is intermediate to the front and rear of said traveling vehicle body; a belt transmission device attached to an upper portion of the mower unit; and a cover unit that covers the belt transmission device; wherein the mower unit includes a blade rotatable with a driven pulley of the belt transmission device, and a mower deck that covers the blade from the above; the cover unit includes a lower cover extending upward from the mower deck at a position outside the belt transmission device, and right and left side covers; the lower cover is attached to the upper portion of the mower unit; the lower cover has an upper end disposed higher than an upper end of the belt transmission device, and when the mower unit is located at its lower limit position, a maintenance space is formed between the lower cover and the lower portion of the traveling vehicle body that is intermediate to the front and rear of said traveling vehicle body; and each of the right and left side covers has a vertical wall, characterized by push rivets and front and rear support arms provided in the right and left side members and detachably attaching the right and left side covers to right and left end portions of the traveling vehicle body via the push rivets, and when the mower unit is located at its lower limit position, a maintenance space is formed between the lower cover and the lower portion of the traveling vehicle body that is intermediate to the front and rear of said traveling vehicle body when the right and left side covers are detached. a traveling vehicle body including a vehicle body frame which includes right and left side members; a mower unit liftably suspension-supported at a lower portion of the traveling vehicle body that is intermediate to the front and rear of said traveling vehicle body; a belt transmission device attached to an upper portion of the mower unit; and a cover unit that covers the belt transmission device; wherein the mower unit includes a blade rotatable with a driven pulley of the belt transmission device, and a mower deck that covers the blade from the above; the cover unit includes a lower cover extending upward from the mower deck at a position outside the belt transmission device, and right and left side covers; the lower cover is attached to the upper portion of the mower unit; the lower cover has an upper end disposed higher than an upper end of the belt transmission device, and when the mower unit is located at its lower limit position, a maintenance space is formed between the lower cover and the lower portion of the traveling vehicle body that is intermediate to the front and rear of said traveling vehicle body; and each of the right and left side covers has a vertical wall, characterized by push rivets and front and rear support arms provided in the right and left side members and detachably attaching the right and left side covers to right and left end portions of the traveling vehicle body via the push rivets, and when the mower unit is located at its lower limit position, a maintenance space is formed between the lower cover and the lower portion of the traveling vehicle body that is intermediate to the front and rear of said traveling vehicle body when the right and left side covers are detached.