The disclosure relates to an organic light emitting display device that can detect a defect in a scan transistor in the device in which the scan transistor and a sensing transistor simultaneously operate, and the device includes: an OLED in a subpixel; a driving transistor connected between the OLED and a driving voltage line; a scan transistor connected between a first node through which a data voltage is applied to the driving transistor and a data line; a sensing transistor connected between a second node between the driving transistor, the OLED, and a reference voltage line; and a defect detector for applying the data voltage in a state in which both the scan transistor and the sensing transistor are turned off and then detecting an amount of charges charged in a parasitic capacitor of the OLED to determine whether the scan transistor is defective due to foreign substances.