An electric connection box comprising: an insulating casing; at least one internal electric part that is accommodated inside the casing and is electrically connected to a plurality of internal electric wires drawn from the inside to outside of the casing; a terminal block that is provided with at least one external electric part electrically connected to at least one external electric wire, and is assembled into an assembly surface in an outer wall of the casing; a first holding structure that is interposed between the assembly surface and a first outer wall surface disposed to face the assembly surface in the terminal block at an assembly completion position, and holds the terminal block on the assembly surface; and a second holding structure that is interposed between an erection wall erected from the assembly surface and a second outer wall surface disposed to face the erection wall in the terminal block at the assembly completion position, and holds the terminal block on the assembly surface, wherein the casing has a held portion configured to be attached to a holding portion of an installation target from an opening of the installation target, characterized in that each of the first holding structure and the second holding structure includes a locking portion (61a, 71a) that is provided on the side of the assembly surface, a locked portion (62a, 72a) that is provided on the side of the terminal block and in which a relative movement toward the side of the opening at the assembly completion position is locked by the locking portion (61a, 71a), a guide portion (61b, 71b) that is provided on the side of the assembly surface and extends along a direction of the relative movement between the locking portion (61a, 71a) and the locked portion (62a, 72a) in a state where the casing is attached to the installation target, and a guided portion (62b, 72b) that is provided on the side of the terminal block and is guided from the side of the opening to the assembly completion position along the guide portion (61b, 71b), the guide portion (61b, 71b) and the guided portion (62b, 72b) being engaged with each other, and at least one of a holding body having the locking portion and the guide portion in the first holding structure and a holding body having the locking portion and the guide portion in the second holding structure is disposed at a position visually recognized by a worker from the opening, in a state where the casing is attached to the installation target. an insulating casing; at least one internal electric part that is accommodated inside the casing and is electrically connected to a plurality of internal electric wires drawn from the inside to outside of the casing; a terminal block that is provided with at least one external electric part electrically connected to at least one external electric wire, and is assembled into an assembly surface in an outer wall of the casing; a first holding structure that is interposed between the assembly surface and a first outer wall surface disposed to face the assembly surface in the terminal block at an assembly completion position, and holds the terminal block on the assembly surface; and a second holding structure that is interposed between an erection wall erected from the assembly surface and a second outer wall surface disposed to face the erection wall in the terminal block at the assembly completion position, and holds the terminal block on the assembly surface, wherein the casing has a held portion configured to be attached to a holding portion of an installation target from an opening of the installation target, characterized in that each of the first holding structure and the second holding structure includes a locking portion (61a, 71a) that is provided on the side of the assembly surface, a locked portion (62a, 72a) that is provided on the side of the terminal block and in which a relative movement toward the side of the opening at the assembly completion position is locked by the locking portion (61a, 71a), a guide portion (61b, 71b) that is provided on the side of the assembly surface and extends along a direction of the relative movement between the locking portion (61a, 71a) and the locked portion (62a, 72a) in a state where the casing is attached to the installation target, and a guided portion (62b, 72b) that is provided on the side of the terminal block and is guided from the side of the opening to the assembly completion position along the guide portion (61b, 71b), the guide portion (61b, 71b) and the guided portion (62b, 72b) being engaged with each other, and at least one of a holding body having the locking portion and the guide portion in the first holding structure and a holding body having the locking portion and the guide portion in the second holding structure is disposed at a position visually recognized by a worker from the opening, in a state where the casing is attached to the installation target.