Scan driving circuit having a number of stages coupled together in series and coupled with first and second lock signal input lines. Each of the stages receives a start signal or an output signal of a previous stage and includes a transfer unit, an inversion unit, and a buffer unit and produces the output signal. The output signal of each of the stages includes a low level signal, the low level signal of each stage is sequentially shifted by one half of the clock signal period with respect to the low level signal of a previous stage. The series of shifted low level signals form the scan signals output by the scan driving circuit for driving an organic light emitting display device.