Angular speed reduction device comprising a first circular element of a fixed plate and meshing with a second element comprising at least one planetary gear and being in contact with the inside of a first ring gear, a third element comprising at least one planetary gear and meshing with the first element and in contact with a second ring gear, in which - the first element traverses the centre of the fixed plate and rotates freely relative to said plate at an angular speed W1, - the second element is rigidly connected to the fixed plate via an end piece integral with said planetary gear and acting as the axis of rotation, each planetary gear being freely rotatable relative to the fixed plate and driven by the first element, the planetary gears being driven by the second element at the angular speed W 2, said ring gear and the first element being concentric, the first ring gear being rigidly connected to a second ring gear so that the first ring gear drives said second ring gear at the same angular speed W 2, one of the two ring gears (4, 5) comprising a skirt without teeth in which the other ring gear is fitted, - the third element meshing with an inner part of the second ring gear, the centre of the third element being driven by the first element and the second ring gear at the angular speed W O3 along a circular trajectory of a given radius intermediate between the radius of the first element and the inner radius of the second ring gear, said planetary gear being mounted so as to be freely rotatable in an output plate via an end piece integral with the planetary gear and acting as the axis of rotation, each planetary gear being freely rotatable about its axis, the third element driving the output plate at the angular speed W 3, - the output plate further comprising at least one end piece acting as the output axis, this axis and the first element being concentric, the components (1, 2, 3, 4, 5, 6, 8, 9) of the device being inserted in a guide tube positioned in the flange or the frame of an engine, the outside diameter of the second ring gear and the diameter of the output plate of the device being equal and adapted to the diameter of the bore of the guide tube so that there is no mechanical play between on the one hand the inner wall of the guide tube and on the other hand the output plate, the third element and the second ring gear, - the guide tube further comprising at the level of its outer end an annular rim designed to receive a mounting plate disposed opposite the output plate and fixed to the guide tube via a least one screw or pin, the mounting plate comprising an opening through which the end piece of the output plate acting as the output axis protrudes, said end piece being freely rotatable relative to the mounting plate. - the first element traverses the centre of the fixed plate and rotates freely relative to said plate at an angular speed W1, - the second element is rigidly connected to the fixed plate via an end piece integral with said planetary gear and acting as the axis of rotation, each planetary gear being freely rotatable relative to the fixed plate and driven by the first element, the planetary gears being driven by the second element at the angular speed W 2, said ring gear and the first element being concentric, the first ring gear being rigidly connected to a second ring gear so that the first ring gear drives said second ring gear at the same angular speed W 2, one of the two ring gears (4, 5) comprising a skirt without teeth in which the other ring gear is fitted, - the third element meshing with an inner part of the second ring gear, the centre of the third element being driven by the first element and the second ring gear at the angular speed W O3 along a circular trajectory of a given radius intermediate between the radius of the first element and the inner radius of the second ring gear, said planetary gear being mounted so as to be freely rotatable in an output plate via an end piece integral with the planetary gear and acting as the axis of rotation, each planetary gear being freely rotatable about its axis, the third element driving the output plate at the angular speed W 3, - the output plate further comprising at least one end piece acting as the output axis, this axis and the first element being concentric, the components (1, 2, 3, 4, 5, 6, 8, 9) of the device being inserted in a guide tube positioned in the flange or the frame of an engine, the outside diameter of the second ring gear and the diameter of the output plate of the device being equal and adapted to the diameter of the bore of the guide tube so that there is no mechanical play between on the one hand the inner wall of the guide tube and on the other hand the output plate, the third element and the second ring gear, - the guide tube further comprising at the level of its outer end an annular rim designed to receive a mounting plate disposed opposite the output plate and fixed to the guide tube via a least one screw or pin, the mounting plate comprising an opening through which the end piece of the output plate acting as the output axis protrudes, said end piece being freely rotatable relative to the mounting plate.