The ISG (Idle Stop & Go) system may include a vehicle information receiving unit receiving a vehicle information, and a control unit including an ISG operation logic which performs an idle stop when a preset idle stop condition is satisfied and restarts the engine when a preset restart condition of the engine is satisfied, and an ISG deactivation determination logic which determines whether a preset ISG deactivation condition is satisfied or not, based on the accumulated number of determinations that the idle stop condition is not satisfied, the accumulated number of idle stops, and the accumulated number of determinations that a performance time of the idle stop is smaller than a preset idle stop retention time, and deactivates the ISG operation logic when the ISG deactivation condition is satisfied.