A fluorescence lifetime measurement was performed for monitoring the viscosity of a confined liquid (glycerol) film between mica surfaces using a cyanine dye, Cy1, as a viscosity probe at various film thicknesses. The fluorescence decay curve consisted of two components, i.e., one attributed to the Cy1 in the central part of the liquid between the surfaces and another to the Cy1 adsorbed on the surfaces. The increased lifetime of the former indicated the increase in the viscosity of the confined liquid.