The purpose of this paper is to present the basic data relating to the reliability of the electrical installation at the large-scale office building. The maintenance record data of A-building，its total floor area is 74,827㎡, and observation term is 1,795days，and B-building 142,759㎡, 3,910days，are obtained. The number of troubles occurred at the electrical installation of A-building and B-building during the observation terms are 214 and 2,515 respectively. At first，the characteristics of the occurrence of troubles of the electrical installation are grasped by comparing with that of the other building equipment. Then the Weibull analysis are adopted to the time between trouble data, and the repair time data are analyzed on logarithmic-normal probability paper. The reliability of the electrical installation of A-and B-building are clarified.