Connecting shaft and manufacturing method of connecting shaft

A connecting shaft, comprising:

• a first shaft made of metal, the first shaft including a serration formed on an outer peripheral surface at an end part thereof, the serration including a first region and a second region;
• a second shaft made of resin, the second shaft including an inner peripheral surface fitting on the first region of the serration at an end part thereof;
• and
• a collar made of metal, the collar including a first fitting portion fitting on the second region of the serration and a second fitting portion fitting on an outer peripheral surface of the second shaft,
• wherein a gap between the outer peripheral surface of the second shaft and an inner peripheral surface of the second fitting portion is filled with an adhesive agent,
• wherein the collar includes a projection projected radially outward from an outer peripheral surface of the collar over a whole circumference thereof, and
• wherein the adhesive agent is applied from the gap up to the outer peripheral surface of the collar between an end face of the second fitting portion in an axial direction and the projection to cover the end face of the second fitting portion.