Topological phase transition, for example, Kosterlitz-Thouless transition, has long been a subject of theoretical physics, but experimental systems exhibiting such an exotic phase transition are quite rare. To find out materials that possibly exhibit a topological phase transition, we performed the magnetic-force microscopy (MFM) measurement for a layered magnet, La5Mo4O16. It is known that this compound exhibits an antiferromagnetic ordering below 200 K, and there are additional phase transitions accompanied by an anomaly in the lattice constants at 90 K and 70 K. We found in the MFM that ferromagnetic domains with the size of ca. 100 nm appear below 70 K, but they disappear with the applied magnetic field of 0.3 T. Together with the large positive magnetoresistance observed below 70 K, it is suggested that the boundaries of the magnetic domains are electrically conducting, possibly due to the topologically protected edge state. This experimental result may suggest that the anomaly at 70 K is caused by some kind of a topological phase transition.