A structure of flexible printed circuit boards (150, 160) comprising: a rectangular substrate; a total of four bar-type flexible printed circuit boards comprised of: a pair of lower flexible printed circuit boards positioned across from each other at opposite edges of the rectangular substrate and mounted with anode pads and cathode pads positioned in both end portions of each lower flexible printed circuit board; and a pair of upper flexible printed circuit boards positioned across from each other at the other two opposite edges of the rectangular substrate relative to the pair of lower flexible printed circuit boards to partially overlap the lower flexible printed circuit boards and mounted with anode pads and cathode pads positioned in both end portions of each upper flexible printed circuit board, wherein the anode pad and the cathode pad of the upper flexible printed circuit board and the anode pad and the cathode pad of the lower flexible printed circuit board, which overlap each other, conduct electricity by a conductive material that fills a first hole formed in a portion of the upper flexible printed circuit board at the anode pad that overlaps the lower flexible printed circuit board and a second hole formed in a portion of the upper flexible printed circuit board at the cathode pad that overlaps the lower flexible printed circuit board. a rectangular substrate; a total of four bar-type flexible printed circuit boards comprised of: a pair of lower flexible printed circuit boards positioned across from each other at opposite edges of the rectangular substrate and mounted with anode pads and cathode pads positioned in both end portions of each lower flexible printed circuit board; and a pair of upper flexible printed circuit boards positioned across from each other at the other two opposite edges of the rectangular substrate relative to the pair of lower flexible printed circuit boards to partially overlap the lower flexible printed circuit boards and mounted with anode pads and cathode pads positioned in both end portions of each upper flexible printed circuit board, wherein the anode pad and the cathode pad of the upper flexible printed circuit board and the anode pad and the cathode pad of the lower flexible printed circuit board, which overlap each other, conduct electricity by a conductive material that fills a first hole formed in a portion of the upper flexible printed circuit board at the anode pad that overlaps the lower flexible printed circuit board and a second hole formed in a portion of the upper flexible printed circuit board at the cathode pad that overlaps the lower flexible printed circuit board. a pair of lower flexible printed circuit boards positioned across from each other at opposite edges of the rectangular substrate and mounted with anode pads and cathode pads positioned in both end portions of each lower flexible printed circuit board; and a pair of upper flexible printed circuit boards positioned across from each other at the other two opposite edges of the rectangular substrate relative to the pair of lower flexible printed circuit boards to partially overlap the lower flexible printed circuit boards and mounted with anode pads and cathode pads positioned in both end portions of each upper flexible printed circuit board, wherein the anode pad and the cathode pad of the upper flexible printed circuit board and the anode pad and the cathode pad of the lower flexible printed circuit board, which overlap each other, conduct electricity by a conductive material that fills a first hole formed in a portion of the upper flexible printed circuit board at the anode pad that overlaps the lower flexible printed circuit board and a second hole formed in a portion of the upper flexible printed circuit board at the cathode pad that overlaps the lower flexible printed circuit board.