A sub-engine type refrigeration system for transportation including an engine for driving a chiller, and an AC generator driven by the engine , wherein all or part of an electric load of the chiller is driven by power generated by the AC generator , the system comprising: a secondary battery which is capable of storing the power generated by the AC generator , and a control unit which controls so that, when a state of charge of the secondary battery is at or above a control lower limit value, the engine is stopped and the chiller's electric load is driven by power of the secondary battery , and when the state of charge of the secondary battery is below the control lower limit value while the chiller's electric load is driven by the power of the secondary battery, the engine is operated and the AC generator starts power generation, whereby the chiller's electric load is driven by the power of the AC generator , and characterised in that the control unit also controls an amount of charge to the secondary battery so as to maintain a load factor of the AC generator substantially at a rated load, wherein the control unit has a function of controlling so that, when the state of charge of the secondary battery is below the control lower limit value while the chiller's electric load is driven by the power of the secondary battery , the engine is operated and the AC generator starts power generation, whereby the chiller's electric load is driven by the generated power from the AC generator , and also controlling a load factor of the engine to 70 to 100% to drive the AC generator and variably controlling the amount of charge to the secondary battery in accordance with variations in power consumption on the chiller side. a secondary battery which is capable of storing the power generated by the AC generator , and a control unit which controls so that, when a state of charge of the secondary battery is at or above a control lower limit value, the engine is stopped and the chiller's electric load is driven by power of the secondary battery , and when the state of charge of the secondary battery is below the control lower limit value while the chiller's electric load is driven by the power of the secondary battery, the engine is operated and the AC generator starts power generation, whereby the chiller's electric load is driven by the power of the AC generator , and characterised in that the control unit also controls an amount of charge to the secondary battery so as to maintain a load factor of the AC generator substantially at a rated load, wherein the control unit has a function of controlling so that, when the state of charge of the secondary battery is below the control lower limit value while the chiller's electric load is driven by the power of the secondary battery , the engine is operated and the AC generator starts power generation, whereby the chiller's electric load is driven by the generated power from the AC generator , and also controlling a load factor of the engine to 70 to 100% to drive the AC generator and variably controlling the amount of charge to the secondary battery in accordance with variations in power consumption on the chiller side. <EOS>