A power transmission shaft, comprising: a male spline formed in an outer periphery thereof; and a diameter increasing portion formed on a part of a tooth bottom of the male spline, which is located on an opposite side to an axial end of the power transmission shaft, by gradually increasing an outer diameter dimension of the tooth bottom, the tooth bottom of the male spline having a substantially constant circumferential width (c) extending up to an axial region of the diameter increasing portion characterized in that the diameter increasing portion comprising a triangular chamfered portion formed in the axial region of the diameter increasing portion, the chamfered portion connecting the tooth bottom and a tooth flank of the male spline to each other to reduce a tooth groove width (d), the tooth bottom, the tooth flank, and the chamfered portion each comprising a press-formed surface, a first connecting ridgeline (a) being formed between the chamfered portion and the tooth flank and a second connecting ridgeline (b) being formed between the chamfered portion and the tooth bottom, a tooth tip of the male spline being reduced in diameter from a position on the first connecting ridgeline (a) as approaching to the opposite side to the axial end so as to be continuous with an outer peripheral surface of the power transmission shaft, the first connecting ridgeline (a) and the second connecting ridgeline (b) intersecting at a first intersection point at the tooth bottom located at an end portion of the diameter increasing portion on the axial end side, this first intersection point corresponding to a diameter increase start position of the male spline, a second intersection point of the first connecting ridgeline (a) and the tooth tip corresponding to a tooth groove width reduction start position, and the tooth groove width (d) being reduced from the tooth groove width reduction start position as approaching to the opposite side to the axial end, a third intersection point of the second connecting ridgeline (b) and the outer peripheral surface of the shaft corresponding to a diameter increase finish position, and the power transmission shaft satisfying a relationship of C>D, where C represents an axial dimension between the diameter increase start position and the tooth groove width reduction start position of the male spline, and D represents an axial dimension between the tooth groove width reduction start position and the diameter increase finish position of the male spline. a male spline formed in an outer periphery thereof; and a diameter increasing portion formed on a part of a tooth bottom of the male spline, which is located on an opposite side to an axial end of the power transmission shaft, by gradually increasing an outer diameter dimension of the tooth bottom, the tooth bottom of the male spline having a substantially constant circumferential width (c) extending up to an axial region of the diameter increasing portion characterized in that the diameter increasing portion comprising a triangular chamfered portion formed in the axial region of the diameter increasing portion, the chamfered portion connecting the tooth bottom and a tooth flank of the male spline to each other to reduce a tooth groove width (d), the tooth bottom, the tooth flank, and the chamfered portion each comprising a press-formed surface, a first connecting ridgeline (a) being formed between the chamfered portion and the tooth flank and a second connecting ridgeline (b) being formed between the chamfered portion and the tooth bottom, a tooth tip of the male spline being reduced in diameter from a position on the first connecting ridgeline (a) as approaching to the opposite side to the axial end so as to be continuous with an outer peripheral surface of the power transmission shaft, the first connecting ridgeline (a) and the second connecting ridgeline (b) intersecting at a first intersection point at the tooth bottom located at an end portion of the diameter increasing portion on the axial end side, this first intersection point corresponding to a diameter increase start position of the male spline, a second intersection point of the first connecting ridgeline (a) and the tooth tip corresponding to a tooth groove width reduction start position, and the tooth groove width (d) being reduced from the tooth groove width reduction start position as approaching to the opposite side to the axial end, a third intersection point of the second connecting ridgeline (b) and the outer peripheral surface of the shaft corresponding to a diameter increase finish position, and the power transmission shaft satisfying a relationship of C>D, where C represents an axial dimension between the diameter increase start position and the tooth groove width reduction start position of the male spline, and D represents an axial dimension between the tooth groove width reduction start position and the diameter increase finish position of the male spline.