A precision planetary gear comprising: a pinion (I; 2; 102) having a first rotation axis, a movable annular gear (IV; 25), a fixed annular gear (III; 9), one or more planet gears (II; 19; 219); the movable annular gear (IV; 25) and the fixed annular gear (III; 9) being coaxial to said first axis; each planet gear (II; 19; 219) having a second rotation axis; said first and second axes being substantially parallel to one another; each planet gear (II; 19; 219) simultaneously meshing with the pinion (I; 2; 102), the fixed annular gear (III; 9) and the movable annular gear (IV; 25); wherein the planet gear (II; 19; 219), the fixed annular gear (III; 9) and the movable annular gear (IV; 25) are toothed annular gears; wherein the number of teeth of the fixed annular gear (III; 9) and of the movable annular gear (IV; 25) is given by the following equation: z 1 = z 2 + n wherein z1 is the number of teeth of the annular gear with the greatest diameter; z2 is the number of teeth of the annular gear with the smallest diameter; and n is the number of planet gears (II; 19; 219); the pinion (I; 2; 102) comprising a gear with a beveloid toothing; each planet gear (II; 19; 219) comprising a gear with a beveloid toothing; wherein the beveloid toothing has a correction, which linearly varies along the longitudinal direction of the teeth of the gear; the precision planetary gear being characterized in that the fixed annular gear (III; 9) and the movable annular gear (IV; 25) are linked to one another by the following equation: x B = x A + LA 2 + v + LB 2 ⋅ tan γ ⋅ π 180 m wherein: LA is the extent along the second rotation axis of the annular gear with the smallest inner diameter; LB is the extent along the second rotation axis of the annular gear with the greatest inner diameter; x B is the value of the correction of the planet gear (II; 19; 219) in the middle of the meshing with the annular gear with the greatest inner diameter; x A is the value of the correction of the planet gear (II; 19; 219) in the middle of the meshing with the annular gear with the smallest inner diameter; v is the extent of the compartment between the fixed annular gear (III; 9) and the movable annular gear (IV; 25) along the second rotation axis; γ is the taper (γ) of the planet gear (II; 19; 219), of the fixed annular gear (III; 9) and of the movable annular gear (IV; 25); and wherein the planet gear (II; 19; 219), the fixed annular gear (III; 9) and the movable annular gear (IV; 25) have the same module m=2R/z wherein 2R is the diameter of the circumference addendum circle of the toothed annular gears; z is the number of teeth of the toothed annular gears. z1 is the number of teeth of the annular gear with the greatest diameter; z2 is the number of teeth of the annular gear with the smallest diameter; and n is the number of planet gears (II; 19; 219); the pinion (I; 2; 102) comprising a gear with a beveloid toothing; each planet gear (II; 19; 219) comprising a gear with a beveloid toothing; wherein the beveloid toothing has a correction, which linearly varies along the longitudinal direction of the teeth of the gear; the precision planetary gear being characterized in that the fixed annular gear (III; 9) and the movable annular gear (IV; 25) are linked to one another by the following equation: LA is the extent along the second rotation axis of the annular gear with the smallest inner diameter; LB is the extent along the second rotation axis of the annular gear with the greatest inner diameter; x B is the value of the correction of the planet gear (II; 19; 219) in the middle of the meshing with the annular gear with the greatest inner diameter; x A is the value of the correction of the planet gear (II; 19; 219) in the middle of the meshing with the annular gear with the smallest inner diameter; v is the extent of the compartment between the fixed annular gear (III; 9) and the movable annular gear (IV; 25) along the second rotation axis; γ is the taper (γ) of the planet gear (II; 19; 219), of the fixed annular gear (III; 9) and of the movable annular gear (IV; 25); and wherein the planet gear (II; 19; 219), the fixed annular gear (III; 9) and the movable annular gear (IV; 25) have the same module m=2R/z wherein 2R is the diameter of the circumference addendum circle of the toothed annular gears; z is the number of teeth of the toothed annular gears.