A plasma processing apparatus includes at least three gas supply lines connected to a process chamber in parallel to allow a gas to flow therethrough, and at least three flow-rate controllers provided on the gas supply lines to detect the flow rate of the gas flowing through each of the flow-rate controllers to control the flow rate to a set value and a diagnosis method. The apparatus has a function of splitting and supplying a gas controlled to a predetermined flow rate by the third flow-rate controller, to a first flow-rate controller for the smallest detectable range of the three flow-rate controllers and to a second flow-rate controller, in order to test the operation of the control of the flow rate of the first flow-rate controller, based on the value obtained from the flow rate of the gas flowing through the second flow-rate controller and the predetermined flow rate.