We succeed in measuring the oscillating force of cylindrical poly(N-isopropylacrylamide) gel having a side group of ruthenium tris(2,2'-bipyridine) (Ru(bpy)(3)) accompanied with the periodic redox reaction of Ru(bpy)(3)(2+/3+), the Belousov-Zhabotinsky (BZ) reaction. The measurements are made for three gels with different diameters, 0.65, 1.17, and 1.88 mm, at 15 degreesC. It is found that the amplitude of oscillatory tensile stress of the reacting cylindrical gel with a diameter of 0.65 mm is much higher than the stress theoretically expected from the Donnan osmotic pressure. The amplitude is explained by an oscillatory change of the interaction parameter, chi, which might be induced by the hydration and dehydration of the chain due to the oscillatory charge density on the polymer chain. The oscillation behavior of the BZ reaction is found to be strongly dependent on the diameter of the gel. The staying period in the oxidative state and the period of the oscillation increase with a decrease in the diameter of the cylindrical gel.