We demonstrate that magnetic domains of ultrathin films can be imaged in situ in ultrahigh vacuum (UHV) by scanning near-field optical microscopy in combination with a Sagnac interferometer (Sagnac-SNOM). In the present set-up, the Sagnac interferometer is not only used for measuring the magneto-optical Kerr effect but also for tip-to-sample distance control. The high sensitivity of the instrument enabled us to study the magnetization reversal processes in externally applied magnetic fields in ultrathin films of Fe and Ni grown on Cu(100) single crystals.