Carrying out a first-principles calculation assuming linear relationship between the electron density at Be nucleus and the electron-capture ͑EC͒ decay rate, we explained why 7 Be@C 60 shows higher EC decay rate than 7 Be crystal, which was originally found experimentally by Ohtsuki et al. ͓Phys. Rev. Lett. 93, 112501 ͑2004͔͒. From the results of the calculation, we found that there are inequivalent four stable ͑i.e., lower energy͒ Be sites inside C 60 and that center of C 60 ͑C_C 60 ͒ is the most favorable site. For C _ C 60 , the electron density at the Be nucleus is the highest. It is also much higher than that at the Be nucleus in a Be crystal. Also, we estimated the expected electron density at the Be nucleus at room temperature by taking statistical average of the electron densities at the four Be nucleus sites using the Boltzmann distribution. The results of the calculation show fairly good agreement with the experimental results. In this paper, we focus on the detail of calculation, which was not fully demonstrated in the paper by Ohtsuki.