Shape memory alloys (SMAs) have been employed to apply to heat-engines using low temperature thermal energy (such as hot exhaust heat and hot spring water). However, since previous SMA heat-engines use wire- or coil-shape SMA elements, it is necessary to increase the applied strain of SMA elements for increasing engine power. Furthermore, a certain part of the SMA element is locally heated and cooled repeatedly in the mechanism adapted by previous engines, which leads to inhomogeneous temperature variation and deformation of the SMA element which causes the overload at that part of the SMA element. Accordingly, the product life of SMA heat engines decreases as engine power increases. Hence SMA heat-engines have not been put into practical use. Then, we produced the heat engine using spiral spring actuators joined by multistage gear (the gear-driven type) to improve product life. However, the output efficiency of this SMA heat-engine is 50 % lower than that of conventional SMA heat-engines. The decrease of efficiency is due to the mechanical loss of multistage gear. Therefore, we produced a new heat-engine using spiral spring actuator joined by a pulley belt (the belt-driven type), and investigate output characteristics of the engine. In consequence, the output efficiency of the new engine is 20 % higher than that of the gear-driven type heat-engine using a spiral spring actuator.