To provide a method for producing high-resistance GaN crystal layers effective by application during the production of a GaN-based field-effect transistor(FET).

This method for producing high-resistance GaN crystal layers 2A and 2B comprises doping at least one kind of a p-type impurity selected from the group of C, Mg and Zn when the GaN crystal is epitaxially grown. Specifically, Mg or Zn is doped in a hydrogen atmosphere at ≥600°C temperature when the GaN crystal is epitaxially grown or the Mg or Zn is doped at ≥1×1017 cm-3 concentration and C is then doped at ≥1×1018 cm-3 concentration when the GaN crystal is epitaxially grown.