A mid-mount riding grass mower includes a belt transmission device (3) attached to an upper portion of a mower unit (2), and a cover unit (4) that covers the belt transmission device (3). The cover unit (4) includes a lower cover (47) extending upward from a position on more traveling vehicle body outer side than the belt transmission device (3) in a mower deck (42), and right and left side covers (48) detachably attached to right and left end portions of the traveling vehicle body (1). The lower cover (47) has an upper end disposed higher than an upper end of the belt transmission device (3), and has a vertical length for forming a maintenance space (49) relative to a front/rear intermediate lower portion of the traveling vehicle body (1) when the mower unit (2) is located at its lower limit position. Each of the right and left side covers (48) has a vertical wall (48A) that is overlapped with the lower cover (47) as seen in a right/left direction in a lifting range of the mower unit (2).
