This paper reports on an improving method of an extracted power of a thermoelectric generator (TEG) by switching the connections in the TEG according to connected device characteristics and ambient conditions such as temperature. We proposed a TEG with a series/parallel switching function, which can switch a combination of series and parallel numbers of the thermoelectric elements (TE elements). By this series/parallel switching function, the power supply characteristics of the TEG can be adjusted to the load characteristics of the connected device and the ambient temperature in order to improve the extracted power to the connected device. In this paper, we designed and fabricated a TEG with a series /parallel switching function. Then, we evaluated the extracted power to the connected device in several ambient conditions.