There is disclosed an apparatus for mounting a grass box of a mower having a machine body, a covered grass box, a mowing unit and a duct through which the grasses mowed by the mowing unit are inputted into the grass box. The apparatus includes a pair of guide rails, laid under the cover, into which the grass box is removably inserted from the rear side. In the mower having the covered grass box disposed below a handle rod, the apparatus further includes stoppers through which the grass box is rotatably removably engaged with the cover and a fixing member for fixing the grass box to the cover in a closed state.