A power transmission shaft comprises a shaft unit interposed between first and second shafts; an engaged portion provided on one end of the second shaft and having an annular stepped part provided between a smaller diameter portion and a larger diameter portion; and an engaging portion provided on one end of the shaft unit and joinable with the engaged portion to rotate together about a common rotation axis, wherein the engaging portion comprises a cylindrical base portion that rotates together with the shaft unit, an elastically shrinkable cylindrical portion that is provided on one axial end of the cylindrical base portion to project toward the engaged portion and catching pawls that are provided by the elastically shrinkable cylindrical portion and latchingly engageable with the annular stepped part when the engaged portion and the engaging portion are coaxially arranged and joined with each other.