This paper deals with the study on the possibility of an aerial flashover between coil conductor and casing in the rotating machine under lower voltage compared with Paschen's voltage even when enough insulation distance was secured. It might be considered that the partial surface flashover generated from microscopic pore of insulating solid ingenerates the aerial flashover. In this study, the flashover voltage was measured using cable which had a simulated microscopic pore to confirm the above consideration. As a result, when the potential gradient existed on the surface of the insulating solid, the flashover voltage decreased compared with that without the potential gradient. It became also clear that the aerial flashover probability is determined mainly by the energy of surface flashover rather than the energy of main aerial flashover.