PURPOSE: To enable effective element isolation, by applying reactive ion etching using a gas composed of carbon and fluorine to a substrate regions between elements on a GaAs substrate on which a plurality of elements are formed.

CONSTITUTION: A mask 2 for selective ion implantation is formed on a GaAs substrate 1, and Si ion or the like is implanted. After annealing, an active layer 3 is formed. A gate electrode 4, a source drain electrode 5 are formed on the active layer region. In order to apply reactive ion etching to regions between elements, an element protecting mask is formed, and the above regions are subjected to reactive ion etching using CF4 gas. Thereby changing stoichiometry of GaAs, and enabling easily element isolation.