Wire connection structure and wire connection method

A harness is provided with one or more first wires each including a first core exposed portion, one or more second wires each including a second core exposed portion, and a bonded portion formed by welding the first and second core exposed portion(s). A total cross-sectional area of the second core(s) is equal to or less than ⅓ of the sum of total cross-sectional areas of the first and second core(s). The bonded portion has two pairs of outer surfaces extending along an extending direction of the first and second wires and facing each other and a distance H between one pair of the outer surfaces (upper/lower surfaces), out of the two pairs of outer surfaces, is longer than a distance W between another pair of the outer surfaces (right/left side surfaces). The second core exposed portion(s) is/are arranged adjacent to the upper surface Up in the bonded portion.