An electric motor assembly for configuring an electric motor, comprising: an electric motor shaft; a rotor which is rotated about the electric motor shaft; a stator disposed around the rotor and including a pair of coil ends which are respective ends in an axial direction of the electric motor shaft; a casing which accommodates the rotor and the stator and allows a sealant composed of a synthetic resin material having a thermal conductivity and an electric insulation property to be injected into the casing in a state that the sealant covers at least the coil ends; a lead wire connected to at least one of the coil ends of the stator and including a portion to be drawn to an outside of the casing; characterised in that it further comprises: a partition wall which separates a sealant space into which the sealant is to be injected from a non-sealant space into which the sealant is not to be injected, the partition wall being formed with a through-hole penetrating through the partition wall so as to bring the sealant space and the non-sealant space into communication with each other; and a plug to be attached to the partition wall so as to close the through-hole, wherein: the plug is made of a material having elasticity higher than elasticity of a material composing the casing; the plug includes an outer peripheral surface tapered from a side of the sealant space toward a side of the non-sealant space and has a tubular shape surrounding a lead-wire insertion hole located in a center of the plug; and the plug is fitted into the through-hole so as to bring the outer peripheral surface of the plug into contact with an inner peripheral surface of the partition wall, the inner peripheral surface surrounding the through-hole, in a state that the lead wire is drawn through the lead-wire insertion hole and one end of both ends of the plug, the one end having a larger diameter of the outer peripheral surface, is located on the side of the sealant space. an electric motor shaft; a rotor which is rotated about the electric motor shaft; a stator disposed around the rotor and including a pair of coil ends which are respective ends in an axial direction of the electric motor shaft; a casing which accommodates the rotor and the stator and allows a sealant composed of a synthetic resin material having a thermal conductivity and an electric insulation property to be injected into the casing in a state that the sealant covers at least the coil ends; a lead wire connected to at least one of the coil ends of the stator and including a portion to be drawn to an outside of the casing; a partition wall which separates a sealant space into which the sealant is to be injected from a non-sealant space into which the sealant is not to be injected, the partition wall being formed with a through-hole penetrating through the partition wall so as to bring the sealant space and the non-sealant space into communication with each other; and a plug to be attached to the partition wall so as to close the through-hole, wherein: the plug is made of a material having elasticity higher than elasticity of a material composing the casing; the plug includes an outer peripheral surface tapered from a side of the sealant space toward a side of the non-sealant space and has a tubular shape surrounding a lead-wire insertion hole located in a center of the plug; and the plug is fitted into the through-hole so as to bring the outer peripheral surface of the plug into contact with an inner peripheral surface of the partition wall, the inner peripheral surface surrounding the through-hole, in a state that the lead wire is drawn through the lead-wire insertion hole and one end of both ends of the plug, the one end having a larger diameter of the outer peripheral surface, is located on the side of the sealant space. the plug is made of a material having elasticity higher than elasticity of a material composing the casing; the plug includes an outer peripheral surface tapered from a side of the sealant space toward a side of the non-sealant space and has a tubular shape surrounding a lead-wire insertion hole located in a center of the plug; and the plug is fitted into the through-hole so as to bring the outer peripheral surface of the plug into contact with an inner peripheral surface of the partition wall, the inner peripheral surface surrounding the through-hole, in a state that the lead wire is drawn through the lead-wire insertion hole and one end of both ends of the plug, the one end having a larger diameter of the outer peripheral surface, is located on the side of the sealant space.