Lock portions 4 are provided more inside than outermost wall surfaces 5a, 6a of a cassette relay block 1, and the cassette relay block 1 is inserted into space surrounded by peripheral walls 16 to 19 on an attaching side, and fixed by the lock portions 4 and locked portions 10 located on the peripheral wall side. The peripheral walls 16 to 19 on the attaching side are formed as a cassette frame 2. The cassette frame is used in common for the cassette relay block 1 or other cassette electric parts mounting blocks. The plural cassette frames are coupled to each other by lock members 54, and coupled to a connection box body by locked members thereby to constitute an electric connection box.