The influence of water sorption on refractive index of a fluorinated polyimide was demonstrated by measuring water uptake and change in volume due to water sorption. The refractive index of the fluorinated polyimide increased with increasing the water uptake. The change as much as 10(-3) in refractive index was observed at the temperature of 25 degreesC. These results reflected a one order smaller change in volume than that in weight due to water sorption. A loose intermolecular packing structure of the fluorinated polyimide mainly induced this small change in volume due to water sorption. We confirmed this dependence of refractive index on water uptake using a Mach-Zehnder Interferometer type switch fabricated using the fluorinated polyimide waveguide.