Ensuring of a proper indoor environment in office buildings requires, among others, the achievement and maintenance of comfort for those abiding in the existing temperature of the air. The paper presents the results of an analysis of the influence of internal heat gains on the consumption of energy in an office building designed in compliance with passive technology. The main energy consuming process in such type of buildings is air-conditioning and cooling. The building was designed as a low-energy one, cooled by means of cooling ceilings. The architect’s and designer’s assumptions were checked by the numerical simulation made by means of the ESP-r software. Basing on the simulation results, the annual energy consumption was analyzed for the different internal heat gains in the office rooms.