In this study, the internal heat load and operation of an HVAC system were investigated in an office drawing low internal heat load where multi-split type air conditioning system were adopted. Field and questionnaire surveys and an energy consumption analysis were performed. The designed total cooling capacity of the HVAC system was 158 W/m2, relatively small compared to most systems that have had a 200 W/m2 capacity. However, the average load factor was below 30%. Moreover, it was found that the operation of a dedicated outdoor air system had a large influence on the total electric energy consumption in offices practicing internal load reduction, and that the main cause for the operation change was the humidity setting of the system. Introducing air-conditioning systems with an appropriate smaller capacity and a comprehensive energy-saving operation method, rather than partially optimizing each device, would be beneficial in terms of total energy saving.