A gas oven, comprising: a body; a cooking cavity formed inside the body; a divider detachably mounted at the cooking cavity to divide the cooking cavity into a first individual cooking cavity at an upper portion of the body and a second individual cooking cavity at a lower portion of the body, wherein the divider is provided with insulation material and is configured to insulate the first individual cooking cavity from the second individual cooking cavity; a first burner provided at the first individual cooking cavity; a second burner provided at the second individual cooking cavity; a supply passage configured to supply gas to the first burner and the second burner from an outside gas supply source, the supply passage comprising a common passage connected to the outside gas supply source, and a first passage and a second passage that are diverged from the common passage to be connected to the first burner and to the second burner, respectively; a main discharging passage to discharge a waste gas from at least the first individual cooking cavity; a first flow rate modulating valve provided at the first passage and capable of modulating an opening degree thereof; a second flow rate modulating valve provided at the second passage and capable of modulating an opening degree thereof; and either an ON/OFF valve provided at the common passage, or a first ON/OFF valve provided at the first passage and a second ON/OFF valve provided at the second passage, characterised by an auxiliary discharging passage to discharge the waste gas from the second individual cooking cavity when the divider is mounted. a body; a cooking cavity formed inside the body; a divider detachably mounted at the cooking cavity to divide the cooking cavity into a first individual cooking cavity at an upper portion of the body and a second individual cooking cavity at a lower portion of the body, wherein the divider is provided with insulation material and is configured to insulate the first individual cooking cavity from the second individual cooking cavity; a first burner provided at the first individual cooking cavity; a second burner provided at the second individual cooking cavity; a supply passage configured to supply gas to the first burner and the second burner from an outside gas supply source, the supply passage comprising a common passage connected to the outside gas supply source, and a first passage and a second passage that are diverged from the common passage to be connected to the first burner and to the second burner, respectively; a main discharging passage to discharge a waste gas from at least the first individual cooking cavity; a first flow rate modulating valve provided at the first passage and capable of modulating an opening degree thereof; a second flow rate modulating valve provided at the second passage and capable of modulating an opening degree thereof; and either an ON/OFF valve provided at the common passage, or a first ON/OFF valve provided at the first passage and a second ON/OFF valve provided at the second passage, characterised by an auxiliary discharging passage to discharge the waste gas from the second individual cooking cavity when the divider is mounted.