It is known that energy use for lighting accounts for about 20% of the total energy use in Japanese typical office buildings. Electrical power saving countermeasures for lighting, such as partial lighting, were taken after the Great East Japan Earthquake and it was indicated that electrical power use for lighting could be substantially reduced. However, the deterioration of the quality of the visual environment of the office workers was concerned at that time. It is required to find a new lighting method that guarantees the quality of the visual environment in addition to reducing electrical power consumption.<br> The purpose of this study is to identify the effects of the lighting control system for ambient lighting with personal switch on energy saving and the occupants' visual comfort. Three different types of the switch for ambient lighting were tested in a real office space located in Chuo-ward Tokyo, Japan. Field measurements were conducted twice, before introducing the personal lighting control system (in June 2012) and after introducing that system (in July 2012), one week for each. Electrical power consumption for ambient lighting, operating conditions of the switch for ambient lighting (on/off) were recorded. The occupancy rate of the occupants, horizontal illuminance and vertical illuminance on window were measured. The lighting environment in the office space and the usability of the switch for ambient lighting were evaluated by the occupants.<br> The results showed that the electrical power consumption for ambient lighting was reduced by about 44% in the whole measured office space and that lasted for the next half year except for the summer vacation. The subjects were rather satisfied with the lighting environment in their task area even the brightness level on the desk was not so high due to the individual control of the ambient lighting. However, it was also identified that the occupants seldom switched off the ambient lighting during working hours even in the cases when they left their seats. One of the reasons of this was supposed that the occupants felt fear for the effects of their ambient lighting control on the surrounding lighting environment. Especially for the ambient lighting that could be controlled by more than one occupant, it was kept switched on even when those who living in the area where was lit by the lighting were absent.<br> In addition, the effect of the number of the occupants who switch on/off the ambient lighting on the frequency of the occupants' control of the ambient lighting was examined. As results, it was identified that the number of the occupants who switch on/off the target ambient lighting should be minimized to promote active individual control of ambient lighting when they left their seats.