A mounting structure for an electrical device, comprising: a mounting main body that supports or houses an electrical device; and a sliding part (3, 4) that is movable in a sliding direction along a bottom surface of said mounting main body by a guide part provided on said bottom surface, wherein said sliding part includes: a main body; a through hole (39, 49) for insertion of a mounting screw; and a sliding engagement part (34, 44) that engages a projecting position locking part (254, 264) formed on said bottom surface, when said sliding part (3, 4) is disposed at a projecting position at which said through hole (39, 49) is located outwardly of a side surface of said mounting main body, and in a state in which said sliding part (3, 4) is disposed at said projecting position and said mounting main body is directly mounted on a mounting surface, both sides of said sliding engagement part (34, 44) in a direction perpendicular to said bottom surface are in direct contact with said projecting position locking part (254, 264) and said mounting surface, characterized in that said projecting position locking part (254, 264) is a groove, said sliding engagement part (34, 44) is provided in a vicinity of a free end of a member (33, 412) supported in a cantilever manner and has a raised portion projecting farther on a side of said bottom surface than said main body in said sliding part (3, 4). a mounting main body that supports or houses an electrical device; and a sliding part (3, 4) that is movable in a sliding direction along a bottom surface of said mounting main body by a guide part provided on said bottom surface, wherein said sliding part includes: a main body; a through hole (39, 49) for insertion of a mounting screw; and a sliding engagement part (34, 44) that engages a projecting position locking part (254, 264) formed on said bottom surface, when said sliding part (3, 4) is disposed at a projecting position at which said through hole (39, 49) is located outwardly of a side surface of said mounting main body, and in a state in which said sliding part (3, 4) is disposed at said projecting position and said mounting main body is directly mounted on a mounting surface, both sides of said sliding engagement part (34, 44) in a direction perpendicular to said bottom surface are in direct contact with said projecting position locking part (254, 264) and said mounting surface, a main body; a through hole (39, 49) for insertion of a mounting screw; and a sliding engagement part (34, 44) that engages a projecting position locking part (254, 264) formed on said bottom surface, when said sliding part (3, 4) is disposed at a projecting position at which said through hole (39, 49) is located outwardly of a side surface of said mounting main body, and in a state in which said sliding part (3, 4) is disposed at said projecting position and said mounting main body is directly mounted on a mounting surface, both sides of said sliding engagement part (34, 44) in a direction perpendicular to said bottom surface are in direct contact with said projecting position locking part (254, 264) and said mounting surface, characterized in that said projecting position locking part (254, 264) is a groove, said sliding engagement part (34, 44) is provided in a vicinity of a free end of a member (33, 412) supported in a cantilever manner and has a raised portion projecting farther on a side of said bottom surface than said main body in said sliding part (3, 4).