Abstract: A diamond film field effect transistor, excellent in characteristics and a method of manufacturing the transistor, in which a contact resistance between electrodes and respective source and drain region is small. The transistor has a channel layer of a p-layer made of semiconducting diamond, an i-layer made of high resistance diamond as a gate insulating layer, which is formed on the channel layer, a gate electrode film formed on the i-layer and a source region and a drain region formed on the surface of the i-layer in self-alignment to the gate electrode film by ion implantation using the gate electrode film, side walls and a protective film as masks.