Disclosed are an SiC semiconductor element and manufacturing method for an SiC semiconductor element in which the interface state density of the interface of the insulating film and the SiC is reduced, and channel mobility is improved. Phosphorus (30) is added to an insulating film (20) formed on an SiC semiconductor (10) substrate in a semiconductor element. The addition of phosphorous to the insulating film makes it possible to significantly reduce the defects (interface state density) in the interface (21) of the insulating film and the SiC, and to dramatically improve the channel mobility when compared with conventional SiC semiconductor elements. The addition of phosphorus to the insulating film is carried out by heat treatment. The use of heat treatment to add phosphorous to the insulating film makes it possible to maintain the reliability of the insulating film, and to avoid variation in channel mobility and threshold voltage.