The invention relates to a device (1) at a mobile grinding machine (2) for stone materials and that comprises a rotatably driven disc-shaped housing (3), which is arranged to support a plurality of grinding disc holders (6) rotatably driven by a belt (5) or chain driven by a motor (4), which holders are distributed along the circumference of said housing (3) and supported by shafts (11) connected to said housing (3) at a mutual distance from each other. According to the invention, internally (7) in a casing (8), which surrounds a said housing (3), a gear rim (10), situated concentrically with a centre driving shaft (9), is arranged non-turnably connected to the casing (8). The grinding disc holder shafts (11) support an intermediate coupling cogwheel (18) each that co-operates non-turnably with the appurtenant shaft and with said gear rim. The gear rim (10) has a through opening (19) for the shaft (9) driven by the motor (4), and between the gear rim (10) and the housing (3), formed of a grinding supporting drum, a seal (31) is arranged that extends along their outer periphery (1OA, 3A). Upon driving actuation of the motor (4), it is accomplished that the grinding disc holders (6) and the housing (3) are driven to rotate in different directions (100 and 200, respectively) in relation to each other.