A friction body for allowing an image having reversible thermochromism to change color from a first state to a second state by frictional heat, wherein the above-mentioned frictional body has a contact area ranging from 0.5 to 15 mm 2 when pressed onto a glass plate with a load of 1000 g, a writing instrument containing a reversible thermochromic ink, which is provided with the above-mentioned friction body, and a writing instrument set having the writing instrument containing a reversible thermochromic ink and the above-mentioned friction body.