A gas detector comprises: plural gas sensors provided with a metal-oxide semiconductor whose resistance changes based upon contact with a gas; and a driving circuit for operating the gas sensors. The gas detector stores the ratio between initial resistance in air of the metal-oxide semiconductor and initial resistance of the metal-oxide semiconductor in an atmosphere including a predetermined concentration of fron gas, for the plural gas sensors. The gas detector learns resistance in air of the metal-oxide semiconductor in a gas sensor in use, and detects occurrence of fron gas when resistance of the metal-oxide semiconductor of the gas sensor in use becomes lower than a value corresponding to the learned resistance in air divided by the ratio. The gas detector counts the period that a first gas sensor is used. When the first gas sensor has been used for a predetermined period, both the first gas sensor and a second gas sensor are used for a learning period to continue detection of fron by the first gas sensor and to learn the resistance in air of the metal-oxide semiconductor of the second gas sensor. After completion of the learning period, fron leakage is detected by the second gas sensor.