A new technique for local magnetic field measurement in a bumpy torus NBT‐1M (which is a magnetic plasma confinement device) has been developed using a rapid‐frequency‐scan (RAFS) laser system combined with sodium atomic beam probing. The RAFS laser beam excites the sodiumD1transition in the plasma cavity, and the Zeeman pattern is determined with laser fluorescence spectroscopy in a single laser shot. From the Zeeman splitting, the transient magnetic field distribution in the plasma, produced by electron cyclotron resonance heating (ECH), is measured with an accuracy of ±3.2%, a spatial resolution of 5 mm, and a temporal resolution of 5 &mgr;s.