We searched for the neutrino-less double beta decay (0νββ) Among double beta decay nuclei, 48 Ca has an advantage of the highest Q ββ -value (4.27 MeV).This large Q ββ -value gives a large phase-space factor to enhance the 0νββ rate and the least contribution from natural background radiations in the energy region of the Q ββ -value. Therefore good signal to background ratio is ensured in the measurement of 0νββ. However not many studies have been carried out so far [6,7,8,9,10], since the natural abundance of 48 Ca is only 0.187 %.We carried out the measurements of 0νββ with CaF 2 (Eu) scintillators. We previously reported a lower limit (90 % confidence level) of 1.4 × 10 22 year for the half life of 0νββ of 48 Ca [1]. The measurement employed the ELEGANT VI system at Oto Cosmo Observatory. We observed 0 event in the Q ββ -value region although expected background exceeded 1 event which limits our experimental sensitivity. In what follows we describe characteristics of our measurement to achieve further background reduction.

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