An insulator for insulating a coil that is wound around a tooth of a stator core from the stator core includes: an axial wall that opposes an axial end surface of the tooth in a state of being assembled to the tooth; and a circumferential wall that is integrally provided with the axial wall and opposes a circumferential-side surface of the tooth. The circumferential wall that is located between the tooth and the coil is constructed of an insulating sheet that is thinner than the axial wall and has foamability.