A phenomenon often appears that the final achieved density is limited during conventional flash sintering even under alternative current electric field. By observing the cross-sectional microstructure of the flash sintered 8 mol %Y 2 O 3 -doped ZrO 2 at this state, we found that a thin surface portion with near full density surrounds a porous inside with lower density. The densified thin surface portion is shell-like structure covering a whole of the sintered compact. The formation of this characteristic shell-like structure in the early stage of flash sintering greatly restricts subsequent densification of porous inside. In order to suppress the formation of the shell that inhibit densification, a technique of controlling a steep power spike at a flash event must be necessary to be appropriately transitioned.