A gas supply apparatus includes: a gas supply conduit for supplying a gas to a digestive tract a pressure detecting device which detects a pressure in the digestive tract a flow regulating device which regulates a gas amount supplied from the gas supply source to the digestive tract via the gas supply conduit a first control device which controls the flow regulating device according to a result detected by the pressure detecting device to supply the gas into the digestive tract so that the pressure inside the digestive tract becomes a set pressure a second control device which controls the flow regulating device to supply a fixed gas amount into the digestive tract and a gas supply mode switching device which switches between gas supply modes including a mode for executing control by the first control device and a mode for executing control by the second control device.
