A polysilicon gate layer, a first n.sup.+ diffusion region serving as a drain region, and a second n.sup.+ diffusion region serving as a source region form a MOSFET, and then an operating point of the MOSFET is set into its saturation region by connecting a gate layer and a drain region of the MOSFET. The first and second n.sup.+ diffusion regions provide a first and a second leakage paths, respectively. A temperature sensor can be provided by use of the event that a leakage current flowing through the second leakage path is varied according to a substrate temperature. According to such configuration, scatter of detected temperatures due to scattering in manufacturing process can be reduced even if all scattering parameters in manufacturing process are considered. In addition, an required area of the temperature sensor can be made smaller since a high resistance value is not needed.