An inter-phase insulating paper is inserted, using a guide jig, into slots formed in the inner circumferential surface of a stator core. The guide jig is disposed at a predetermined position on one end surface side of the stator core. The inter-phase insulating paper is inserted into the guide jig along the stator core central axis line. The guide jig deforms the inter-phase insulating paper to a predetermined shape when the inter-phase insulating paper passes through the inside of the guide jig. The guide jig inserts both side portions of the inter-phase insulating paper simultaneously into different two slots of the stator core along the stator core central axis line.