An electrical junction box comprising: a box main body that contains and holds electrical components; a cover member that covers an opening in said box main body; a locking mechanism that fits and locks peripheral walls of said box main body and said cover member on each other; and a fitting projection piece provided with one of said box main body and said cover member, said fitting projection piece protrudes from an end of one of said peripheral walls, and said fitting projection piece is fitted into a side of an inner peripheral surface of the other peripheral wall of the other of said box main body and said cover member, wherein the other of said box main body and said cover member is provided on at least a single of side portions of said other peripheral wall with said locking mechanism, and an elastic rib that protrudes from said inner peripheral surface of said other peripheral wall at said single side portion is pressed onto said fitting projection piece, and said other of said box main body and said cover member is provided on the other side portion of said other peripheral wall, which is not provided with said locking mechanism, with a support wall that is opposed to and spaced apart from said inner peripheral surface of said other peripheral wall, and said fitting projection piece is held in a space between said inner peripheral surface of said other peripheral wall and said support wall, characterised in that a pair of elastic ribs (40a, 40b) that are opposed to each other, is provided on said inner peripheral surface of said other peripheral wall and a surface of said support wall opposed to said other peripheral wall, and said fitting projection piece is held in a space between protruding end surfaces of said pair of elastic ribs (40a, 40b). a box main body that contains and holds electrical components; a cover member that covers an opening in said box main body; a locking mechanism that fits and locks peripheral walls of said box main body and said cover member on each other; and a fitting projection piece provided with one of said box main body and said cover member, said fitting projection piece protrudes from an end of one of said peripheral walls, and said fitting projection piece is fitted into a side of an inner peripheral surface of the other peripheral wall of the other of said box main body and said cover member, wherein the other of said box main body and said cover member is provided on at least a single of side portions of said other peripheral wall with said locking mechanism, and an elastic rib that protrudes from said inner peripheral surface of said other peripheral wall at said single side portion is pressed onto said fitting projection piece, and said other of said box main body and said cover member is provided on the other side portion of said other peripheral wall, which is not provided with said locking mechanism, with a support wall that is opposed to and spaced apart from said inner peripheral surface of said other peripheral wall, and said fitting projection piece is held in a space between said inner peripheral surface of said other peripheral wall and said support wall, characterised in that a pair of elastic ribs (40a, 40b) that are opposed to each other, is provided on said inner peripheral surface of said other peripheral wall and a surface of said support wall opposed to said other peripheral wall, and said fitting projection piece is held in a space between protruding end surfaces of said pair of elastic ribs (40a, 40b).