A high-Tc superconducting cable(HTS cable) is expected as an underground power line supplying the electrical power the densely populated city in future. The electrical insulation is very important for develop HTS cable system because it is operated a high voltage and in cryogenic temperature. We manufactured a mini-model cable and measured a tan$delta$ of cable using schering bridge. The tan$delta$ of PPLP was lower than that of Tyvek and Kraft at a given temperature, the tan$delta$ of PPLP was 1.16${ imes}$10-3. According to the increase of electric stress the tan$delta$ increased because partial discharge occurred inside butt gap of mini-model cable. However, the tan$delta$ decreased by increase of liquid nitrogen pressure. This reason is thought by decrease of part discharge between butt gap by increase of liquid nitrogen pressure.