The vortex flows in the wake of a circular cylinder were visualized by hydrogen bubble techniques. The images of the flow past a bubble generating-wire frozen by a flashing light were consecutively exposed on a moving roll-film in a camera to save the waiting time for the next exposure. A method of image processing for the automatic measurement of the temporal velocity profiles crossing the generating-wire was developed in the digital-images of the flow captured in a micro computer. The successive data of the temporal velocity profiles were transformed into the data in space domain by use of Taylor-hypothesis, and the procedure to determine space derivative of the velocity, i.e., the velocity gradient tensor was established to investigate the kinematic structure of the vortices in the wake.