Radiation pressure exerted on a single microparticle within an evanescent field has been analyzed by a three-dimensional potential measurement system. Temporal fluctuation of the particle position due to the thermal Brownian motion was sequentially measured, and the potential energy was estimated by a thermodynamical analysis. A potential profile corresponding to the evanescent-field-induced radiation pressure could be obtained as an energy difference between two potentials measured with and without illumination of the evanescent field. The exponential dependence on the longitudinal position was clearly observed. The lateral radiation force on a 4.5 μm polystyrene latex particle in water was determined to be 90 fN at the laser intensity of 1.0 kW cm-2.