Air flow rate controlling system and air flow rate controlling method

A flow rate controlling system includes: a flow rate balance determining portion that determines a supply flow rate, regulated by a supply air valve, and a common exhaust flow rate, regulated by a common exhaust air valve, so that a difference between the supply flow rate and an exhaust flow rate, regulated by the common exhaust air valve, matches a set point; a valve monitoring portion that monitors the supply air valve, the common exhaust air valve, and a local exhaust air valve; and a flow rate changing portion that issues a flow rate changing instruction to the flow rate balance determining portion. When there is a fault in the supply air valve and/or the common exhaust air valve, the flow rate balance determining portion, upon receipt of the flow rate changing instruction from the flow rate changing portion, changes neither the supply nor common exhaust flow rate.