Disclosed is a gas sensor for detecting hydrogen gas in a measurement gas atmosphere, including: a first installation part defining a first inner space in communication with the measurement gas atmosphere through a solid electrolyte membrane member; a second installation part defining a second inner space in direct communication with the measurement gas atmosphere; first and second sensor elements respectively installed in the first and second inner spaces; and a calculation unit configured to calculate the concentration of the hydrogen gas according to a potential between the first and second sensor elements. The gas sensor further includes a current detecting portion that detects a current flowing through the first and second sensor elements. The calculation unit is configured to, when the current detected by the current detecting portion is larger than or equal to a threshold value, judge that the hydrogen gas is present at high concentration.