This paper presents the design, fabrication, and testing procedures of a compliant auto-focusing device for a microscope. To obtain a long focusing range, a new flexure positioning mechanism is devised based on multi-stage leaf springs. The proposed device allows a precise focusing without friction and backlash effects. A voice coil motor is employed to drive the positioning mechanism. The stiffness model and resonant-frequency model are developed analytically. These models are employed for an architectural optimization of the mechanism parameters to maximize the resonant frequency under the driving stroke and force constraints. The performance of the designed flexure mechanism is validated through finite-element analysis (FEA) simulation investigations. Results show that the positioning mechanism enables a long focusing range over 10 mm with a resonant frequency higher than 50 Hz, which allows an auto-focusing operation with rapid response. Moreover, a prototype is fabricated for experimental studies. LabVIEW software with Vision Development Module is adopted for image processing in the auto-focusing process. By employing the standard deviation of the image grayscale as the criterion of focusing, an exhaustive search method is used to determine the focusing point. Experimental results demonstrate the effectiveness of the developed auto-focusing device.