We would like here to show in the SU~-symmetr ic Lee model tha t we can der ive the correct sign of the e lec t romagnet ic mass difference of nucleons if we t r ea t in a self-consistent manner the s y m m e t r y breaking due to the e lec t romagnet ic in teract ion. Since the success of DASltEN and FRAVTSCHI (~) in calculat ing the n-p mass difference by assuming tha t nucleons are composi te , some a t t e m p t s (") h a v e been made oa clar i fying the connect ion be tween the i r approach and the conven t iona l ones (a) af ter the work of ]?I~YNMAN and SPEXS.~IA.~" (4). I t has been suggested t h a t in the f ie ldtheoret ical approach i t is not necessary to assume nucleons to be composi te and the correct va lue of the n-p mass difference m a y be obta ined by t ak ing into account the effect of the nucleon mass shift back on itself. In the present paper we shah p rove in the exac t ly soluble model t h a t the above-ment ioned po in t of v iew is in fact t rue. All of the works before tha t of DASHE.~ and FRAUTSCItl (1) h a v e considered in calcula t ing the n-p mass difference only the one-photon con t r ibu t ion to the electromagne t ic mass shif t of nucleons as shown in Fig . 1 (a). These works have g iven the wrong sign of the n-p mass difference because of the dominance of the Coulomb in teract ion over the magnet ic one at low energies. In these approaches the effect of the e lec t romagnet ic mass shif t of the v i r tua l nucleon as shown in Fig. 2 was