Gases are smoothly and automatically switched when using two gas supply sources, that is, an air pump and a carbon dioxide gas cylinder. By using a gas supply flow rate adjusting unit, a carbon dioxide gas is supplied to an endoscope, and a gas using a carbon dioxide gas and water are delivered. When the remaining amount of the carbon dioxide gas of the cylinder is detected and the detected pressure of the carbon dioxide gas becomes less than a predetermined value, the air pump is rotated to supply pressurized air. The rotation number of the air pump is controlled, and the same amount of pressurized air is supplied as that of the supplied carbon dioxide gas. The switching to the pressurized air is automatically performed. The supply amounts before and after the switching become equal to each other, and a surgical operator does not feel any discomfort.
