A jacket having a pair of small helical gears is spline-fitted around a splined shaft. Since the small helical gears have an adverse helix angle to each other, the jacket moves along the spline shaft in the axial direction so as to distribute an equal torque to both small helical gears. The splited torque is transmitted to a pair of drive shafts each of which drives the main rotor of a helicopter through a combination of gear and pinion. The drive shafts are fixed axially and therefore helical gears or double-helical gears can be used in the combination of gear and pinion. Consequently the power transmission apparatus according to the present invention provides a light and compact structure,