In order to clarify the acceleration mechanism of applied-field magneto-plasma-dynamic arcjet (MPDA) plasma, the spatial profiles of the flow field and electromagnetic field near the outlet of the MPDA were measured using magnetic probes and the spectroscopic method. The plasma current densities and Lorentz forces acting on the plasma were evaluated experimentally. It was found that the azimuthal rotation of the exhausted plasma in the applied magnetic field is determined by a balance among the E×B drift, the diamagnetic drift, and the centrifugal force drift. Three components of the Lorentz force, i.e., the radial, the azimuthal, and the axial, were measured experimentally for the first time. The radial component Fr was dominant among the three components and the axial one (Fz) was weakened by the deceleration force, which spontaneously appeared in the applied-field MPDA plasma due to a diamagnetic effect of the high-beta plasma. It was demonstrated that the deceleration force can be converted to an acceleration force in an externally applied diverging field.