We report the temperature-induced volume phase transition behavior of a copolymer gel of N-isopropylacrylamide and sodium acrylate (NIPA-AA) at pH 9.5 in the range of low mole fraction of the acrylate x (x < 0.01), either in salt-free aqueous solution or NaCl aqueous solutions. Introduction of electric charges into the network switched the temperature-induced volume change of the gel from a discontinuous to a continuous type in salt-free aqueous solutions. The switching of the regime took place within a very narrow range of x around a very small value of 0.002. The continuous volume change in salt-free aqueous solution was transformed into the discontinuous one in NaCl aqueous solutions of 0.1 and 0.3 M. Effects of the two factors, the charged monomer fraction and the ionic strength, were explained in a unified manner in terms of the Donnan osmotic pressure. We found that the continuous or the discontinuous regime of NIPA-AA gel was well characterized, respectively, by the high or low osmotic pressure. Similar results were obtained with NIPA-sodium poly(styrene sulfonate) copolymer gels.