A temperature measuring sensor is incorporated in a substrate of a semiconductor device and has a diode, and resistor formed in the substrate and connected in series. When a first forward constant current is supplied to the diode through the resistor, a potential difference VA1 is produced between terminal ends of both the diode and the resistor, and a potential difference VF1 is produced between terminal ends of the diode. When a second forward constant current is supplied to the diode through the resistor, a potential difference VA2 is produced between the terminal ends of both the diode and the resistor, and a potential difference VF2 is produced between the terminal ends of the diode. A real temperature T of the substrate is calculated by the following formula:
T=(q/k)(VF1−VF2)[1/[ln((VA1−VF1)/(VA2−VF2))]]
herein: T is an absolute temperature, k is Boltzmann's constant, and q is an electron charge.