Patent ID: 8435125

Claim:
A power transmission shaft, comprising: a torque-transmission teeth portion formed on an outer peripheral surface on an end portion side; and a smooth portion formed on the outer peripheral surface adjacent to the torque-transmission teeth portion so as to have a stepwise reduced diameter, wherein: a ratio of a minimum diameter of the smooth portion to a minimum diameter of the torque-transmission teeth portion is set to 0.9 or more and 1.02 or less; a carbon content of a raw material of the power transmission shaft is set to 0.37 wt % or more and 0.44 wt % or less; the torque-transmission teeth portion and the smooth portion undergo heat treatment so as to form a heat-treated hardened layer; the heat-treated hardened layer increases in depth from a side of the torque transmission teeth portion to a side of the smooth portion; at least a minimum diameter portion of the smooth portion undergoes full hardening for forming the heat-treated hardened layer from a surface to a central axis; and an average prior austenite grain size of the heat-treated hardened layer is set to 8 μm or more and 35 μm or less.