Thermomagnetic writing of single‐crystal MnBi films has been investigated. The magnetic state of the spot heated with a laser beam is reversed by the stray field resulting from the surrounding saturated film. The reversed spot serves as a magnetic nucleus from which magnetic domains emanate by wall motion due to the demagnetizing field. Thus the magnetic state of the film switches from the remanent saturated state to the demagnetized state of lower energy. The reverse domain does not penetrate into the adjacent crystals beyond the potential barriers of the crystal boundary.