A power steering driving device 100 includes an input shaft 6 provided with a serration 8 and connected to an input mechanism 3 linked to a steering handle 2, an output shaft 7 provided with a serration 10 and connected to an output mechanism 5 linked to a vehicle wheel 4, and a torsion bar 15 that connects the input shaft 6 and the output shaft 7 coaxially. The serration 8 of the input shaft 6 and the serration 10 of the output shaft 7 are formed such that a ratio between the respective numbers of teeth thereof is a decimal, and to specify a relative rotation position of the input shaft 6 and the output shaft 7 relative to the input mechanism 3 and the output mechanism 5, a tooth on the serration 8 of the input shaft 6 and a tooth on the serration 10 of the output shaft 7 having a relative position deviation angle about an identical central axis which is equal to or smaller than an allowable value are selected as reference teeth.