Direct observation of individual T4 DNA molecules during steady field gel electrophoresis was carried out with fluorescence microscopy. Statistical analyses of the image data show that (i) DNA molecules change their conformation and velocity periodically even under a steady field, (ii) the characteristic time of the periodic motion decreases with an increase of the electric field and with a decrease of the gel concentration, and (iii) the rear end of DNA shows a periodic stick-slip motion, while the front end moves with almost constant speed. A possible interpretation for the periodic behavior is presented.