Discharge plasma evolution of DBD plasma actuator was numerically simulated in the three dimensional space to analyze the spanwise non-uniformity in the body force field and discuss the validity of the 2D analysis. A sinusoidal applied voltage was simulated and the simulation could successfully reproduce the characteristics of micro-discharge as reported by previous experimental studies. The body force field obtained in the simulation was spanwise non-uniform and strong spanwise force was also generated even from the time-average view point. The spanwise-averaged body force field was compared with the 2D simulation result. The comparison indicated that the 2D simulation slightly underestimated the chordwise extension of the force field and the force amplitude due to weaker electric field concentration in the 2D plasma structure. However, the discrepancies are small and the body force field in the 2D simulation qualitatively corresponded well to the 3D simulation resuls from the time-average and spanwise-average view point. Therefore, it can be expected that the 2D simulation is sufficiently useful to study qualitative characteristics and the physics in DBD plasma actuator. On the other hand, the 3D simulation can reproduce the spanwise non-uniformity in the body force field, and the effects of the spanwise non-uniformity on flow control can be investigated by inputing the 3D simulation result to CFD simulations.