[PROBLEMS] A gear box for decanter centrifugal separators, which provides a preferred rotational difference between a bowl and a screw conveyor (3) by eliminating problems of a bearing. [MEANS FOR SOLVING THE PROBLEMS] The gear box has a first gear unit, a second gear unit, a first a gear unit, and a transmission carrier. The first gear unit has a first ring gear (R gear) rotated by first drive means, and also has a first planetary gear (P gear) meshing with a non-rotating first sun gear (S gear). The second gear unit has a second P gear meshing with a second R gear that is rotated by the first drive means and also meshing with a second S gear rotated with the revolution of the first P gear, and the second gear unit transmits the revolution of the second P gear to the screw conveyor (3). The first a gear unit has a first aP gear and a first aR gear and transmits the revolution of the first P gear to the second S gear. The first aP gear meshes with a first aS gear connected to a second drive means and revolves about the first aS gear. The first aR gear is rotated with the revolution of the first aR gear and is connected to the second S gear. The transmission carrier supports the revolution of the first and first aP gears. A bearing retainer for the first and first aP gears is a roller guide retainer formed of a heat resistant resin.