We have found the reliable synthetic method of the pi-electron donor molecule BEDT-TSeF. The key step in our synthesis is the isolation of the reactive 1,4-ethylenedithio-5,6-diselenolate dianion as its titanocene complex which easily reacts with triphosgene to give 4,5-ethylenedithio-1,3-diselenol-2-one. The crystal of BEDT-TSeF is isostructural with that of BEDT-TTF. A variety of highly conductive cation radical salts (especially, theta and kappa-type salts) were obtained by electrochemical crystallization with various counter anions.