We address a BEMS (Building Energy Management System) to guarantee reliability of electric-power supply in dynamic uncertain environments. The building microgrid as the target of BEMS has multiple distributed power sources including a photo-voltaic power system and Electric-Vehicle (EV). EV is regarded as an autonomously-moving battery due to the original means of transportation and is hence a cause of dynamic uncertainty of the building microgrid. The main objective of synthesis of BEMS in this paper is to guarantee the continuous supply of power to the most critical load in a building microgrid and to realize the power supply to the other loads according to a ranking of load importance. We synthesize the BEMS as a reactive control system that monitors changes of dynamic uncertain environment of the microgrid including departure and arrival of an EV, and determines a route of power supply to the most critical load. Also, we conduct numerical experiments of the reactive BEMS using models of power flows in the building and of charging states of the batteries. The experiments are incorporated with data measured in a practical office building and demonstration project of EMS at Osaka, Japan. We show that the BEMS works for extending the time duration of continuous power supply to the most critical load.