1. A method of controlling an air separator comprising: calculating a product quantity that can be collected by use of a predetermined maximum raw air quantity that can be supplied and by use of an operating condition of a bubble tower; comparing an input value of a necessary product quantity with said product quantity that can be collected; setting said necessary product quantity as a product formation quantity when said necessary product quantity is smaller than, or equal to, said product quantity that can be collected; outputting said product formation quantity as a set value for a product formation quantity adjusting device; calculating a necessary raw air quantity by use of said set value for said product formation quantity adjusting device and said operating condition of said bubble tower; outputting said necessary raw air quantity as a set value for a raw air quantity adjusting device; setting said product quantity that can be collected as said product formation quantity when said necessary product quantity is greater than said product quantity that can be collected; outputting said product formation quantity as a set value for a product formation quantity adjusting device; and outputting said maximum raw air quantity as a set value for a raw air quantity adjusting device.