A gas concentration detecting device includes a gas concentration detecting element and an electronic control unit. The gas concentration detecting element includes a first electrochemical cell and a second electrochemical cell. The electronic control unit is configured to detect the concentration of the sulfur oxide contained in the test gas based on a first detected value correlated with a current flowing through the first electrochemical cell acquired when a first removing voltage is applied to the second electrochemical cell and a measuring voltage is applied to the first electrochemical cell.