A solder joint structure connecting a flexible printed circuit board and a second board composed of a material that has less flexibility than the flexible printed circuit board, wherein: the flexible printed circuit board includes a plurality of electrode pads formed along an end part of the flexible printed circuit board, each of the plurality of electrode pads includes two corresponding conductive electrodes having generally the same shape on both faces sandwiching the flexible printed circuit board, the two corresponding conductive electrodes are mutually connected by at least one through hole, the two corresponding conductive electrodes, including an inner part of the through hole, have a solder layer formed thereon, and the two corresponding conductive electrodes are further mutually connected by a side face electrode formed on a side face of the end part of the flexible printed circuit board; the second board includes a plurality of electrode pads formed along an end part of the second board to face the conductive electrodes on one face of the flexible printed circuit board and a side face electrode formed continuously from the plurality of facing electrode pads on the second board onto a side face of the end part of the second board and ending in the middle of the side face; and the conductive electrodes on the one face of the flexible printed circuit board and the plurality of facing electrode pads on the second board are solder joined with the solder layer, the solder joint is configured such that a joined state of a soldered portion on the side face electrode of the flexible printed circuit board formed continuously from the solder joint is visible from the second board side of the solder joint, and the solder joint is configured such that a joined state of a soldered portion on the side face electrode of the second board is visible from the flexible printed circuit board side of the solder joint. the flexible printed circuit board includes a plurality of electrode pads formed along an end part of the flexible printed circuit board, each of the plurality of electrode pads includes two corresponding conductive electrodes having generally the same shape on both faces sandwiching the flexible printed circuit board, the two corresponding conductive electrodes are mutually connected by at least one through hole, the two corresponding conductive electrodes, including an inner part of the through hole, have a solder layer formed thereon, and the two corresponding conductive electrodes are further mutually connected by a side face electrode formed on a side face of the end part of the flexible printed circuit board; the second board includes a plurality of electrode pads formed along an end part of the second board to face the conductive electrodes on one face of the flexible printed circuit board and a side face electrode formed continuously from the plurality of facing electrode pads on the second board onto a side face of the end part of the second board and ending in the middle of the side face; and the conductive electrodes on the one face of the flexible printed circuit board and the plurality of facing electrode pads on the second board are solder joined with the solder layer, the solder joint is configured such that a joined state of a soldered portion on the side face electrode of the flexible printed circuit board formed continuously from the solder joint is visible from the second board side of the solder joint, and the solder joint is configured such that a joined state of a soldered portion on the side face electrode of the second board is visible from the flexible printed circuit board side of the solder joint.