In order to obtain guidelines for the design and operation of a new plasma source by a magnetic neutral loop discharge, electron behavior was studied experimentally and numerically. Experimentally, the magnetic field gradient was changed over a wide range, and it was found that there existed an optimum value for efficient plasma production. Analyses of the electron behavior were performed using a model that included effects of a three-dimensional electromagnetic field with spatial decay of the rf electric field, and the limitation of the spatial extent of the electron motion (the existence of walls and the electron loss at wall surfaces). These factors were found to explain the existence of the optimum magnetic held gradient.