A slot insulator is provided and includes: an annular part; and a plurality of insulated winding parts disposed a circumference of the annular part. Each of the insulated winding parts has an insulated winding holding part and an insertion groove for housing a winding part of a tooth in a stator yoke. The insertion groove opens in one direction along a center axis of the annular plate and opens at both ends of the insertion groove in a radial direction of the annular plate.