Friction body unit and thermochromic writing instrument

A friction unit, comprising a friction piece and a retention portion, the friction piece being configured to allow generation of frictional heat to cause handwriting in thermochromic ink to undergo a thermochromic change and the retention portion being configured to allow retention of the friction piece and attachment to and detachment from a barrel or cap of a thermochromic writing instrument, wherein

• the retention portion has a tubular side wall forming an internal space opening at least downward,
• the side wall has at least one vent hole formed to communicate with the internal space, and
• air is allowed to flow between the vent hole, the internal space, and the downward opening,
• the tubular side wall of the retention portion includes a portion with a large outer diameter and a portion with a small outer diameter, and
• the vent hole has a first opening formed on an outer surface of the side wall and a second opening formed on an inner surface of the side wall and at least the first opening is located above a maximum outer diameter portion of the side wall, and
• at least part of the tubular side wall of the retention portion is provide with a tapered portion with an outer diameter decreasing upward from bottom and the first opening of the vent hole is formed in the tapered portion.