A sulfur dioxide detector embodying an SO₂-oxidant bioagent comprises a bio-membrane (40) located at one end of an oxygen concentration sensor (2), the bio-membrane (40) consisting of the bioagent (e.g. Thiobacillus thiooxidons, Thiobacillus ferroxidons or an enzyme devided from such a microorganism) provided between an immobilized membrane (41a) and a gas permeable membrane (41b). The sensor (2) has an electrode (60) in contact with the immobilized membrane (41a) of the bio-membrane (40). A dish-shaped cell (32) has an open end which is in contact with the gas permeable membrane (41b) so that sulfurous acid in an SO₂-containing solution supplied to the cell (32) during use is oxidized to sulfuric acid so as to change the output from the sensor (2). An inlet hole (33) and an outlet hole (34) are formed on a sidewall of the cell (32) with the outlet hole (34) located directly above the inlet hole (33) so that foam produced during use, when the SO₂-containing solution is supplied to the dish-shaped cell (32) through the inlet hole (33), can be drained from the outlet hole (34).