A gas oven having a gas supplying passage to supply gas to a burner is provided. The gas oven includes an ON/OFF valve to open/close the gas supplying passage, and a flow rate modulating valve connected to the ON/OFF valve in series so as to change the cross-sectional area of a passage, thereby supplying a proper amount of gas to a cooking cavity in response to the change of a cooking area. When a cooking cavity is divided into individual cooking cavities while a divider is mounted, the flow rate modulating valve is open/closed at a smaller opening degree than the maximum operating degree, and thus may be able to prevent a cooking substance from being carbonized in the individual cooking cavities.