An alcohol concentration sensor provided a tube shaped means (21) of which the top end (211) is closed with a top end closing means (20) and of which the bottom end (212) is remained open, a gas supply tube means (22) which penetrates the top end closing means (20) to extend down into the tube shaped means (21), which is provided an air blower means (24), and of which the lower end (221) terminates slightly upper than the bottom end (212) of the tube shaped means (21), thereby the lower end (221) of the gas supply tube means (22) is soaked into a liquid (10) containing water and alcohol, of which the alcohol concentration is required to be determined, deeply sufficient to allow the air bubbles flown out of the gas supply tube means (22) into the liquid (10) to absorb sufficient quantity of alcohol contained in the liquid (10), whenever the tube shaped means (21) is soaked in the liquid (10 ), a gas drawing out means (26) which penetrates the top end closing means (20), a gas temperature sensing means (27) which is connected with the gas drawing out means (26) for determining the temperature of the gas in the neighborhood of a gas oxidation means for oxidizing the alcohol evaporated from the liquid (10), a temperature/alcohol concentration calibration means for calibrating the temperature of the gas toward the corresponding alcohol concentration, and an alcohol concentration output means for outputting the alcohol concentration determined by the temperature/alcohol concentration calibration means.