In a plasma generator having a first supply port and a second supply port, in which the plasma generator includes a first supply device for supplying a first gas from the first supply port, and a second supply device connected to the first supply device so as to supply a second gas from the second supply port, a gas supply determination method includes a first supply step of causing the first supply device to start supplying the first gas in a state where the second supply device stops supplying the second gas, a first measurement step of measuring a flow rate of the gas supplied to the first supply port after the first supply step, and a first notification step of notifying a supply state of the first gas in the first supply port in accordance with a flow rate measurement result of the first measurement step.