The electro-osmotic velocity has been measured on a poly(N-isopropylacrylamide) hydrogel plate at various temperatures. The results are analyzed on the basis of a theory for the electro-osmosis on a "soft" surface (i.e., a polyelectrolyte-coated surface) to determine the charge density and the "softness" of the hydrogel layer. It is found that as the temperature is raised, the charge density increases and the softness of poly(N-isopropylacrylamide) hydrogel decreases. These changes are most remarkable around 33 degrees C. This is probably because poly(N-isopropylacrylamide) hydrogel shrinks above its LCST (33 degrees C) and swells below it. This behavior is in good agreement with our previous results obtained from electrophoretic mobility measurements for poly(N-isopropylacrylamide)-coated latex particles. The obtained values of the charge density in the hydrogel layer are compared with those that are estimated by potentiometric titration and the swelling ratio measurement of the hydrogel plate. Agreement is excellent, suggesting that the electro-osmosis measurements should be appropriate for analyzing the physicochemical properties of a soft surface.