Soft member attachment structure and thermochromic writing instrument

A soft-member attachment structure for attaching a soft member to a thermochromic writing instrument, the soft member being used to cause thermochromic ink adhered to a paper surface to undergo a thermochromic change by frictional heat, the soft-member attachment structure comprising:

• the soft member;
• an attachment hole provided through a rear end portion of a barrel or an apex of a cap, the barrel and the cap configuring the thermochromic writing instrument, along a longitudinal central axis and having an inner circumferential surface between two openings located at an upper end and a lower end;
• an attachment portion located under a large diameter portion of the soft member used for a thermochromic change and having a diameter being smaller than a diameter of the large diameter portion and allowing insertion into the attachment hole;
• a straight internal hole provided along a longitudinal central axis of the soft member and opening at least at a lower end of the soft member;
• and
• a rod-like inner core having an outer diameter allowing insertion into the internal hole, a length to be placed within the internal hole, and an outer circumferential surface contacting an inner circumferential surface of the internal hole, wherein
• the attachment portion is inserted into the attachment hole and the inner core is inserted into the internal hole to configure the soft member to be attached to the attachment hole
• wherein in a process of inserting the inner core into the internal hole where the internal hole is a hole with one closed end opening at the lower end of the soft member and not opening at an upper end of the soft member, the inner core is provided with a vent portion to discharge air in the internal hole.