PURPOSE: To enable efficient analysis at low cost by successively updating, storing detailed data for one or several days satisfying conditions, and transferring the updated and stored detailed data to a center on the final day of a set period.

CONSTITUTION: Daily detailed real-time data are collected by a data collector 1 provided for each building, a detailed data file for one day is prepared, and daily report data 5 of almost 1/20 is prepared based on detailed data for that day. Next, the total energy quantity of that day is compared with the total energy quantity of first and second largest detailed data 3 and 4 in the past, for example, and when it is larger, the detailed data 3 and 4 of the first or second smallest total energy are replaced with the detailed data 2 of that day. The daily report data 5 for that day are transferred to the data collector 1 at the center by utilizing a telephone line, and the first or second maximum detailed data 3 and 4 are transferred to the data collector 1 at the center on the final day of the period set to a summer/winter/intermediate period.