For super high precision non-contact measurement of miniature cavities, an optical focus detection system is proposed based on a differential confocal microscopy. This system combines a high axial resolution and absolute position traceability of a confocal microscopy with a small gradient-index lens. The differential confocal principle and the system configuration are presented. In this system, the small detector with 1 mm diameter is obtained by a gradient-index lens, the noise produced by the light source intensity drift and the detector electronic drift is eliminated, and signal-to-noise ratio and axial resolution are increased by differential design of two detectors. Measurement made with 20° degree gauge block, with a detector of 1 mm diameter, axial resolution is up to 5 nm.