A manufacturing method for a power transmission mechanism including: first and second shafts having first and second double helical gears; first and second rolling bearings rotatably supporting the shafts with respect to a case and restrict movement of the shafts in an axial direction thereof, includes an assembling step of assembling an outer ring of the first rolling bearing and an outer ring of the second rolling bearing to the case in a state where the outer rings are movable in the respective axial directions; and a positioning step of determining axial positions of the first rolling bearing and the second rolling bearing while rotating the first shaft and the second shaft in a state where the first double helical gear and the second double helical gear are meshed with each other, after the assembling step.