Magnetic force microscopy (MFM) is a useful tool for investigating magnetic recording media, ferromagnetic patterns and magnetic devices. Ultra-thin magnetic coating probes were developed with the aim of providing high-resolution MFM. High-sensitivity MFM system with quality factor auto control (QFAC) was developed for use with such thin magnetic coating probes. We studied performance of the QFAC system in air and in vacuum. As a result, we found this is a powerful technique for imaging a magnetic domain. The QFAC in vacuum was superior to that in air. We observed finer magnetic domains than those observable with a conventional MFM system. This paper, we reports experimental results for ultra-high-density recording media; we estimate the lateral resolution is less than 20 nm.