Discharge characteristics in single-phase equipment are a well-known and well-established art, however, those in three-phase equipment are far less understood. Because the electric field in the insulating material of three-phase equipment is not linear, but rotates with power frequency, it can be expected that the discharge characteristics in three-phase equipment will be different from those in single-phase equipment. In this paper, the effects of rotating electric field on partial discharge pattern in a three-phase construction are reported. The simplified model of a three-phase GIS with air inside is used. The electric field in the construction was analyzed, and the discharge characteristics were observed. The electric field vector locus in a three-phase construction is elliptical, although the shape differs locally. The elliptical nature of rotating electric field is expressed as an electric field ratio. The experimental results showed that the phase widths where partial discharge current pulses appeared were different depending on the electric field ratio even for the same applied voltage. The phase width where partial discharge current pulses appeared was also influenced by the applied electric field. These results are analysed from the relationship between the value and the locus of the applied electric field and the critical electric field of air.