A gas delivery system which can deliver a predetermined gas into a body cavity of a subject is provided with: an automatic gas delivery conduit through which gas is automatically delivered into the body cavity so that the inside of the body cavity has a predetermined pressure; a manual gas delivery conduit through which gas is delivered into the body cavity by manual operation; a function selection button which is provided to an endoscope; and a control unit which operates in association with the operation of the function selection button and which is adapted to set a gas delivery conduit selected by the function selection button to a connected state and to set a not-selected gas delivery conduit to a blocked state.