An optical box comprising: - a housing, - a retaining device arranged in the housing and comprising at least two substantially parallel seats, wherein the seats are configured to house optical fiber units (11a, 11b) with friction between each optical fiber unit (11a, 1 1b) and the relevant seat, wherein the retaining device is configured to house optical fiber units arranged in at least two columns and two superimposed rows, and wherein the friction between each optical fiber unit and the corresponding seat is substantially the same in two superimposed rows wherein the retaining device comprises: - a comb having retaining walls, the seats being defined by two adjacent retaining walls; and - a cap configured to be removably attached to the comb to maintain substantially constant the size of each seat, wherein the comb comprises a base wall, the retaining walls projecting from the base wall along a first direction (X-X), the retaining walls being arranged mutually spaced apart along a second direction (Y-Y) transverse to the first direction (X-X) to define seats between two adjacent retaining walls, the retaining walls being elastically deformable along the second direction (Y-Y), each seat being configured to retain one or more optical fiber units arranged therein along a third direction (Z-Z) transverse to the first direction (X-X) and the second direction (Y-Y), in superimposed relationship in the first direction (X-X), characterized in that, the cap being configured to engage each retaining wall of the comb to restrain elastic deformation of the retaining walls. - a housing, - a retaining device arranged in the housing and comprising at least two substantially parallel seats, wherein the seats are configured to house optical fiber units (11a, 11b) with friction between each optical fiber unit (11a, 1 1b) and the relevant seat, wherein the retaining device is configured to house optical fiber units arranged in at least two columns and two superimposed rows, and wherein the friction between each optical fiber unit and the corresponding seat is substantially the same in two superimposed rows wherein the retaining device comprises: - a comb having retaining walls, the seats being defined by two adjacent retaining walls; and - a cap configured to be removably attached to the comb to maintain substantially constant the size of each seat, wherein the comb comprises a base wall, the retaining walls projecting from the base wall along a first direction (X-X), the retaining walls being arranged mutually spaced apart along a second direction (Y-Y) transverse to the first direction (X-X) to define seats between two adjacent retaining walls, the retaining walls being elastically deformable along the second direction (Y-Y), each seat being configured to retain one or more optical fiber units arranged therein along a third direction (Z-Z) transverse to the first direction (X-X) and the second direction (Y-Y), in superimposed relationship in the first direction (X-X), characterized in that, the cap being configured to engage each retaining wall of the comb to restrain elastic deformation of the retaining walls. wherein the seats are configured to house optical fiber units (11a, 11b) with friction between each optical fiber unit (11a, 1 1b) and the relevant seat, wherein the retaining device is configured to house optical fiber units arranged in at least two columns and two superimposed rows, and wherein the friction between each optical fiber unit and the corresponding seat is substantially the same in two superimposed rows wherein the retaining device comprises: - a comb having retaining walls, the seats being defined by two adjacent retaining walls; and - a cap configured to be removably attached to the comb to maintain substantially constant the size of each seat, - a comb having retaining walls, the seats being defined by two adjacent retaining walls; and - a cap configured to be removably attached to the comb to maintain substantially constant the size of each seat, wherein the comb comprises a base wall, the retaining walls projecting from the base wall along a first direction (X-X), the retaining walls being arranged mutually spaced apart along a second direction (Y-Y) transverse to the first direction (X-X) to define seats between two adjacent retaining walls, the retaining walls being elastically deformable along the second direction (Y-Y), each seat being configured to retain one or more optical fiber units arranged therein along a third direction (Z-Z) transverse to the first direction (X-X) and the second direction (Y-Y), in superimposed relationship in the first direction (X-X), characterized in that, the cap being configured to engage each retaining wall of the comb to restrain elastic deformation of the retaining walls.