In the transformer insulation system, the oil gap is subdivided by pressboards. In DC insulation, higher field is applied to the pressboard having higher resistance compared with that of oil. There are a lot of pressboard joints in an actual insulated structure. The high field inside the pressboard transfers to other pressboard at the joint. This field transference causes high field concentration on creepage surface along pressboard. This higher field could trigger a creepage discharge. Regarding the field transference phenomena, we change condition of the pressboard joint and numerically calculate the field intensity caused at the joint in this paper. Discharge field strength is formulated by analyzing results of dielectric breakdown tests measured so far. These results allow us to offer an insulation design method about the field transference.