The effects of surfactants having various chemical structures on the phase transition of poly(N-isopropylacrylamide) (NIPA) gel have been studied. The phase transition temperature of NIPA gel, which is 34 degrees C in pure water, is elevated by several tens of degrees when a small amount of some surfactant is added to water. The transition temperature increases with surfactant concentration below the critical micelle concentration and is saturated at a certain temperature. The transition temperature strongly depends on the chemical structure in the hydrophilic part as well as the length of the hydrophobic portion of the surfactants. The transition temperature of the gel does not depend on the counterion species. These hydrophile-lipophile balance (HLB).