The present invention relates to a reversibly thermochromic aqueous ink composition comprising: water; a water-soluble organic solvent; a reversibly thermochromic microcapsule pigment which contains a reversibly thermochromic composition comprising: (A) an electron donating coloring organic compound, (B) an electron accepting compound, and (C) a reaction medium which determines temperature at which color reactions between the components (A) and (B) occur; a comb type polymer dispersant having carboxyl groups on its side chains, an organic nitrogen sulfur compound, and a water-soluble resin, a writing instrument using the ink composition, and a writing instrument set comprising the writing instrument and a frictional body.