Abstract: A gas determination method uses a sensor having a field-effect transistor structure including a gate electrode, an insulating film formed on the gate electrode, a source electrode and a drain electrode formed on the insulating film, and a graphene layer formed on the insulating film and connecting the source electrode and the drain electrode to each other. The gas determination method includes: supplying gas to the graphene layer; applying a first voltage to the gate electrode for a predetermined period of time; and thereafter measuring a change in a current flowing between the source electrode and the drain electrode when a sweep voltage is applied to the gate electrode, repeating the same current measurement with a second voltage different from the first voltage, and determining the gas based on the measurement results.