To form a strong and compact whole structure by supporting a uniaxial quarrying drum on an extension arm/projecting part by a slender gear housing arranged in its center.

A peripheral wall part 4 becomes wide to both sides in a rear part of a housing 1, and is connected there to a connecting plate 5 for mutual connection, and forms a cover for a driving means such as a motor 18 installed on a level of a driving pinion 20 on the outside of a side plate 2 together with a protective plate 10. A hydraulic radial piston motor 18 is installed on a support hub 9 by a flange, and is connected to drive a shaft 20a of the driving pinion 20 supported on a conical roller bearing 34. A sufficiently large opening part to receive the conical roller bearing 34 is arranged in a support hub 8 and the side plate 2 to insert the driving pinion 20. An intermediate wheel 22 having a large speed reduction gear is inserted through a rear opening part of the housing 1 together with a preassembled roller bearing 12, and is supported on a support stud 13.