A method for manufacturing a semiconductor device is provided in which a semiconductor element that generates heat during operation is formed in an active region of a semiconductor substrate and a temperature sensitive diode sensor arranged to detect temperature is formed in a temperature sensitive diode region of the semiconductor substrate. The method includes: forming a polysilicon layer that composes the temperature sensitive diode sensor in the temperature sensitive diode region, forming a mask, and introducing impurities through the mask into the semiconductor substrate and the polysilicon layer. The mask has an element pattern having an element opening through which a region composing the semiconductor element is exposed in the active region, a diode pattern having a diode opening through which a portion of the temperature sensitive diode region is exposed, and a monitoring pattern provided within the diode pattern with a size smaller than that of the diode opening.