The MEG experiment has been searching for the lepton avor violating process,

+ ! e+

, which is predicted to occur at a sizable rate by many of new physics models

beyond the Standard Model. The upgrade expriment (MEG II) is now in preparation to

obtain one order higher branching ratio sensitivity  4  10􀀀14 by using the world's most

intense muon beam up to  108+=s at Paul Scherrer Institut.

In the MEG II experiment, Radiative Decay Counter (RDC) will be newly intro-

duced to improve the sensitivity by identifying high energy 

-ray (>48 MeV) from radiative

muon decay (RMD : + ! e+e

). This 

-ray is one of the major sources of the back-

ground. According the simulation result, RDC is able to reduce background 

-ray events

by 54%. The improvement of the nal sensitivity is about 28 %.