The present invention provides a method for manufacturing a gallium nitride semiconductor epitaxial crystal substrate with a dielectric film which has a low gate leak current and negligibly low gate lag, drain lag, and current collapse characteristics. The method for manufacturing a semiconductor epitaxial crystal substrate is a method for manufacturing a semiconductor epitaxial crystal substrate in which a dielectric layer of a nitride dielectric material or an oxide dielectric material in an amorphous form functioning as a passivation film or a gate insulator is provided on a surface of a nitride semiconductor crystal layer grown by metal organic chemical vapor deposition. In the method, after the nitride semiconductor crystal layer is grown in an epitaxial growth chamber, the dielectric layer is grown on the nitride semiconductor crystal layer in the epitaxial growth chamber.