A method for calculating concentration of detection target gas comprising: (a) introducing a detected gas containing two gas components and one detection target gas in a gas sensor element and calculating an interim concentration of the detection target gas; (b) introducing the two gas components in a reference sensor element having a same property as a property of the gas sensor element, heating the reference sensor element to a temperature at which both of the two gas components react, and maintaining the temperature for a predetermined period to measure an electrical resistance value of the reference sensor element concurrently with (a), wherein the gas sensor element and the reference sensor element are metal oxide based gas sensor elements; (c) heating the reference sensor element to a temperature at which only any of the two gas components reacts, and maintaining the temperature for a predetermined period to measure an electrical resistance value of the reference sensor element; (d) calculating a concentration ratio of the two gas components based on a combination of the electrical resistance value in (b) and the electrical resistance value in (c); (e) correcting a calibration curve of the two gas components in the gas sensor element and a calibration curve of the two gas components in the reference sensor element preliminarily prepared based on the concentration ratio; (f) using the calibration curve in the reference sensor element corrected in (e) to calculate a concentration of the two gas components at the electrical resistance value in (b); (g) using the calibration curve in the gas sensor element corrected in (e) to convert the concentration of the two gas components calculated in (f) into a concentration of the detection target gas; and (h) calculating a difference between the interim concentration of the detection target gas calculated in (a) and the concentration of the detection target gas converted in (g). (a) introducing a detected gas containing two gas components and one detection target gas in a gas sensor element and calculating an interim concentration of the detection target gas; (b) introducing the two gas components in a reference sensor element having a same property as a property of the gas sensor element, heating the reference sensor element to a temperature at which both of the two gas components react, and maintaining the temperature for a predetermined period to measure an electrical resistance value of the reference sensor element concurrently with (a), wherein the gas sensor element and the reference sensor element are metal oxide based gas sensor elements; (c) heating the reference sensor element to a temperature at which only any of the two gas components reacts, and maintaining the temperature for a predetermined period to measure an electrical resistance value of the reference sensor element; (d) calculating a concentration ratio of the two gas components based on a combination of the electrical resistance value in (b) and the electrical resistance value in (c); (e) correcting a calibration curve of the two gas components in the gas sensor element and a calibration curve of the two gas components in the reference sensor element preliminarily prepared based on the concentration ratio; (f) using the calibration curve in the reference sensor element corrected in (e) to calculate a concentration of the two gas components at the electrical resistance value in (b); (g) using the calibration curve in the gas sensor element corrected in (e) to convert the concentration of the two gas components calculated in (f) into a concentration of the detection target gas; and (h) calculating a difference between the interim concentration of the detection target gas calculated in (a) and the concentration of the detection target gas converted in (g).