A novel method for simultaneous measurement of vertical and lateral forces with high sensitivity and accurate gap control is presented. It combines a doubleended tuning fork resonator for vertical force measurement with an optical fiber probe for lateral force measurement. High force sensitivity of several tens of nano Newons with accurate gap control of less than 1 nm was successfully attained. Simultaneous measurement using this novel method revealed that viscoelasticity increased with decrease of the sliding gap. Moreover, the method first revealed that the lubricant pressure generated by the wedge effect can increase due to the confinement at small gaps, even if the lubricant pressure is so low that the viscosity rise due to the pressure is much small. This method is expected to be useful for clarifying the lubrication phenomena at nanometer sliding gaps, which are not yet fully understood due to the difficulty of measurement.

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