PROBLEM TO BE SOLVED: To provide a semiconductor element for manufacturing a high quality GaSb semiconductor crystal having a flat surface on a semi-insulating substrate; and provide a manufacturing method of the semiconductor element.SOLUTION: A semiconductor element manufacturing method comprises the steps of: increasing a substrate temperature to 550°C under reduced pressure of 0.1 atmosphere in a hydrogen atmosphere; and stopping phosphine supply when the substrate temperature is increased to 550°C and simultaneously supplying triethylgallium (TEGa), arsin (AsH) and trimethyl antimony (TMSb). At this time, a hydrogen bubbling flow rate of triethylgallium is 170 sccm and constant. On the other hand, an arsin flow rate and a hydrogen bubbling flow rate of trimethyl antimony is monotonously changed from 2.3 to 0 sccm, and from 86.7 to 173.4 sccm, respectively, by spending 30 minutes. The semiconductor element manufacturing method further comprises the steps of: subsequently performing growth of GaSb for 30
