A method to measure the absolute intensity of a weak magnetic field is described. This method uses patterned superconducting strip lines as a spatial modulator of the field and detects the magnetic field distribution over them as a function of applied magnetic fields. The field distribution change measured by an rf SQUID system is proportional to the component of the absolute intensity of the magnetic field at the specimen that is parallel with the pick‐up coil axis. The magnetic field intensity can be determined absolutely from about 1 &mgr;G by this method. A spatial resolution of the field as good as a few millimeters is obtained. Some practical limitations of this method and possible ways for improving are also discussed.