In a gas supply system, a controller is electrically connected to a pressure regulator and an operation switch. The controller is operative to control the pressure regulator so that a predetermined gas is supplied to a first delivery member directing to a first body cavity with its pressure regulated to a first pressure suitable for the first body cavity. The controller is operative to control the pressure regulator depending on an instruction sent from the operation switch upon operation of the operator so that the predetermined gas is supplied to a second delivery member directing to a second body cavity with its pressure regulated to a second pressure suitable for the second body cavity.