We have developed a new scanning magnetic microscope to image with micrometric resolution magnetic fields originating from room temperature polished samples. This microscope is based on a giant magnetoresistance (GMR) sensor working at room temperature. These magnetic sensors are sensitive to the in-plane components of the magnetic field. The size of the sensing element is 9 μm × 36 μm. The noise of the GMR sensor is dominated by a low-frequency 1/f noise. The field equivalent noise of the sensors is 10 nT at 1Hz and decreases to 0.3 nT above 1 kHz for a 1 mA sensing current. The spatial resolution of the system is ∼20 μm, and its peak-to-peak noise during operation is ∼250 nT. Its high spatial resolution and a minimum sensor-to-sample distance of 30 μm compensate for its rather moderate field sensitivity. This room temperature small-sized and rugged magnetic microscope appears as a powerful instrument for small-scale rock magnetic investigations.