An organic light emitting display device comprising: a panel driver; and a display panel including a plurality of pixels, each of the plurality of pixels having a pixel circuit that has a driving transistor, a first driving voltage terminal connected to the driving transistor, a light emitting element, a second driving voltage terminal connected to the light emitting element, and a capacitor (Cst) connected between a gate and source electrode of the driving transistor, each of the plurality of pixels connected with a dummy line (Mi), wherein the panel driver is configured to drive the pixel circuit in: a data charging period in which a difference voltage between a data voltage (Vdata) and a reference voltage (Vref) is charged into the capacitor (Cst), and the driving transistor simultaneously receives a first driving voltage having a first voltage level from the first driving terminal, and a light emitting period in which the driving transistor receives the first driving voltage having a second voltage level from the first driving voltage terminal and is turned on according to the voltage charged into the capacitor (Cst) during the data charging period, whereby a current is supplied to the light emitting element connected between the driving transistor and the second driving voltage terminal and the light emitting element thereby emits light; the panel driver being configured to supply the data voltage (Vdata) and the reference voltage (Vref) to the plurality of pixels at the data charging period, and configured to simultaneously change the level of the first driving voltage or a second driving voltage supplied to the plurality of pixels via the first driving voltage terminal and the second driving voltage terminal, respectively, at the data charging period, wherein the first voltage level is lower than the second voltage level and equal to or lower than the reference voltage (Vref), characterized in that the panel driver is configured to drive the pixel circuit in a detection mode: an initialization period in which a data voltage for detection and a pre-charging voltage are charged into the capacitor, the data voltage for detection has a pre-determined voltage level for detecting a threshold voltage of the driving transistor; a detection voltage charging period in which a voltage corresponding to the threshold voltage of the driving transistor is charged into the dummy line by flowing a current by a voltage charged into the capacitor in the initialization period; a voltage detecting period in which the panel driver detects the voltage charged into the dummy line, converts the detected voltage into detection data. a panel driver; and a display panel including a plurality of pixels, each of the plurality of pixels having a pixel circuit that has a driving transistor, a first driving voltage terminal connected to the driving transistor, a light emitting element, a second driving voltage terminal connected to the light emitting element, and a capacitor (Cst) connected between a gate and source electrode of the driving transistor, each of the plurality of pixels connected with a dummy line (Mi), wherein a data charging period in which a difference voltage between a data voltage (Vdata) and a reference voltage (Vref) is charged into the capacitor (Cst), and the driving transistor simultaneously receives a first driving voltage having a first voltage level from the first driving terminal, and a light emitting period in which the driving transistor receives the first driving voltage having a second voltage level from the first driving voltage terminal and is turned on according to the voltage charged into the capacitor (Cst) during the data charging period, whereby a current is supplied to the light emitting element connected between the driving transistor and the second driving voltage terminal and the light emitting element thereby emits light; an initialization period in which a data voltage for detection and a pre-charging voltage are charged into the capacitor, the data voltage for detection has a pre-determined voltage level for detecting a threshold voltage of the driving transistor; a detection voltage charging period in which a voltage corresponding to the threshold voltage of the driving transistor is charged into the dummy line by flowing a current by a voltage charged into the capacitor in the initialization period;