Junction box to be mounted on motor vehicle

A junction box to be mounted on a motor vehicle, comprising

• a casing assembly including an upper casing and a lower casing;
• a laminated unit including bus bars and insulation plates;
• first and second printed circuit boards contained in said casing assembly, said first printed circuit board configured as a higher current circuit having thick-film conductors, and said second printed circuit board configured as a middle or lower current circuit having thin-film conductors with thicknesses smaller than those of said thick-film conductors on said first printed circuit board;
• and
• board relays mounted on a part of different peripheral portions on both sides of said first printed circuit board or a part of a peripheral portion of either of the sides of said first printed circuit board, said board relays disposed in said casing assembly opposed to at least one of a first heat generating element containing space defined between a front side of said first printed circuit board and an inner surface of a top wall of said upper casing, and a second heat generating element containing space defined between a rear side of said first printed circuit board and an inner surface of a bottom wall of said lower casing, wherein
• at least one of said upper casing and said lower casing is provided with a relay containing section for a plug-in type relay to be connected to said bus bars, and said relay containing section is arranged in a position separated away from a position opposed to a mounting position of said board relays.