A flow cell type anion sensor comprising: a flow channel; an ion-sensitive membrane obtained by performing coating treatment on a membrane containing a compound which has a cation, with a material containing a compound which has an ionization group of predetermined pKa; an internal solution accommodation portion which is disposed so as to enable contact of a portion or the entirety of a first surface of the ion-sensitive membrane with a sample and accommodates an internal solution so as to come into contact with a portion or the entirety of a second surface of the ion-sensitive membrane; and an electrode provided so as to come into contact with the internal solution accommodated in the internal solution accommodation portion, wherein the ion-sensitive membrane, the internal solution accommodation portion, and the electrode are disposed in a container, the internal solution contains a pH buffer solution using 2-(Cyclohexylamino)ethanesulfonic acid or an ammonium chloride buffer solution, and NaCl or KCl as a base, the concentration of the chlorine ions in the internal solution is equal to or greater than 1 mmol/L, and pH of the internal solution is adjusted so as to be equal to or greater than pKa of the compound having the ionization group, characterized in that pH of the internal solution is adjusted to a range equal to or greater than pH 7.6, wherein the pH of the internal solution is adjusted by using an ammonium chloride buffer solution if the pH of the internal solution having pH 11.0 is adjusted and in that pH of the internal solution is adjusted by using a 2-(Cyclohexylamino)ethanesulfonic acid if the pH of the internal solution having pH 7.6 and pH 9.6 is adjusted. a flow channel; an ion-sensitive membrane obtained by performing coating treatment on a membrane containing a compound which has a cation, with a material containing a compound which has an ionization group of predetermined pKa; an internal solution accommodation portion which is disposed so as to enable contact of a portion or the entirety of a first surface of the ion-sensitive membrane with a sample and accommodates an internal solution so as to come into contact with a portion or the entirety of a second surface of the ion-sensitive membrane; and an electrode provided so as to come into contact with the internal solution accommodated in the internal solution accommodation portion, wherein the ion-sensitive membrane, the internal solution accommodation portion, and the electrode are disposed in a container, the internal solution contains a pH buffer solution using 2-(Cyclohexylamino)ethanesulfonic acid or an ammonium chloride buffer solution, and NaCl or KCl as a base, the concentration of the chlorine ions in the internal solution is equal to or greater than 1 mmol/L, and pH of the internal solution is adjusted so as to be equal to or greater than pKa of the compound having the ionization group, characterized in that pH of the internal solution is adjusted to a range equal to or greater than pH 7.6, wherein the pH of the internal solution is adjusted by using an ammonium chloride buffer solution if the pH of the internal solution having pH 11.0 is adjusted and in that pH of the internal solution is adjusted by using a 2-(Cyclohexylamino)ethanesulfonic acid if the pH of the internal solution having pH 7.6 and pH 9.6 is adjusted.