An electric wire connection structure comprising: one or more copper-based conductor covered electric wires having a copper-based conductor covered part in which a copper-based conductor made of copper or a copper alloy is covered with an insulating cover and a copper-based conductor exposed part in which a part of the insulating cover is stripped by a predetermined length to expose the copper-based conductor; and one or more aluminum-based conductor covered electric wires having an aluminum-based conductor covered part in which an aluminum-based conductor made of aluminum or an aluminum alloy is covered with an insulating cover and an aluminum-based conductor exposed part in which a part of the insulating cover is stripped by a predetermined length to expose the aluminum-based conductor, wherein an ultrasonic joint part is provided at a conductor stacked part in which the copper-based conductor exposed part and the aluminum-based conductor exposed part are superposed, the electric wire connection structure being characterized in that a joint interface of the ultrasonic joint part satisfies a relational expression of x / L × 100 ≤ 10 % where a total contacting length which is a summed length of a part at which the copper-based conductor exposed part and the aluminum-based conductor exposed part contact is L and a summed length of a contour line of a space formed at a part at which the copper-based conductor exposed part and the aluminum-based conductor exposed part are separate is x based on cross-section observation, the joint interface of the joint part is provided by the aluminum-based conductor covered electric wire being the only one plastically deformed or melted in a gap unjoined which is formed between the copper-based conductor exposed part and the aluminum-based conductor exposed part forming the joint interface so as to fill the gap of the copper-based conductor covered electric wire based on cross-section observation wherein a difference in tensile strength between the copper-based conductor and the aluminum-based conductor is 100 MPa or more. one or more copper-based conductor covered electric wires having a copper-based conductor covered part in which a copper-based conductor made of copper or a copper alloy is covered with an insulating cover and a copper-based conductor exposed part in which a part of the insulating cover is stripped by a predetermined length to expose the copper-based conductor; and one or more aluminum-based conductor covered electric wires having an aluminum-based conductor covered part in which an aluminum-based conductor made of aluminum or an aluminum alloy is covered with an insulating cover and an aluminum-based conductor exposed part in which a part of the insulating cover is stripped by a predetermined length to expose the aluminum-based conductor, wherein an ultrasonic joint part is provided at a conductor stacked part in which the copper-based conductor exposed part and the aluminum-based conductor exposed part are superposed, the electric wire connection structure being characterized in that a joint interface of the ultrasonic joint part satisfies a relational expression of x / L × 100 ≤ 10 % where a total contacting length which is a summed length of a part at which the copper-based conductor exposed part and the aluminum-based conductor exposed part contact is L and a summed length of a contour line of a space formed at a part at which the copper-based conductor exposed part and the aluminum-based conductor exposed part are separate is x based on cross-section observation, the joint interface of the joint part is provided by the aluminum-based conductor covered electric wire being the only one plastically deformed or melted in a gap unjoined which is formed between the copper-based conductor exposed part and the aluminum-based conductor exposed part forming the joint interface so as to fill the gap of the copper-based conductor covered electric wire based on cross-section observation wherein a difference in tensile strength between the copper-based conductor and the aluminum-based conductor is 100 MPa or more. where a total contacting length which is a summed length of a part at which the copper-based conductor exposed part and the aluminum-based conductor exposed part contact is L and a summed length of a contour line of a space formed at a part at which the copper-based conductor exposed part and the aluminum-based conductor exposed part are separate is x based on cross-section observation, the joint interface of the joint part is provided by the aluminum-based conductor covered electric wire being the only one plastically deformed or melted in a gap unjoined which is formed between the copper-based conductor exposed part and the aluminum-based conductor exposed part forming the joint interface so as to fill the gap of the copper-based conductor covered electric wire based on cross-section observation wherein a difference in tensile strength between the copper-based conductor and the aluminum-based conductor is 100 MPa or more.