A rotatable connector device, comprising: a fixed case including a ring-shaped fixed-side ring plate and an outer cylinder having a cylindrical shape and extending from an outer circumferential edge of the fixed-side ring plate perpendicularly to a planar surface of the fixed-side ring plate; and a rotatable case including a ring-shaped rotatable-side ring plate and an inner cylinder having a cylindrical shape and extending from an inner circumferential edge of the rotatable-side ring plate perpendicularly to a planar surface of the rotatable-side ring plate toward the fixed case; and the fixed case and the rotatable case are fit to each other so as to be rotatable with respect to each other in a clockwise direction and a counterclockwise direction; an accommodation section is defined by the fixed-side ring plate and the outer cylinder of the fixed case and the rotatable-side ring plate and the inner cylinder of the rotatable case; in the accommodation section, flat cables for electrically connecting the fixed case and the rotatable case to each other are accommodated in a wound state; the fixed-side ring plate and the outer cylinder are formed of different members; a fixed-side ring plate rotation regulation section is provided at the outer circumferential edge of the fixed-side ring plate; an outer cylinder rotation regulation section is provided at an outer circumferential edge of the outer cylinder, at a position corresponding to the fixed-side ring plate rotation regulation section; and the fixed-side ring plate and the outer cylinder are aligned so as to have the same rotation axis and are fit to each other in a direction of the rotation axis, and the fixed-side ring plate rotation regulation section is put into contact with the outer cylinder rotation regulation section so as not to rotate in either the clockwise direction or the counterclockwise direction about the rotation axis; wherein: the fixed-side ring plate includes a plurality of engaging hole sections each having an engaging hole, the plurality of engaging hole sections projecting perpendicularly from the outer circumferential edge toward the outer cylinder and located at a prescribed interval in a circumferential direction; the outer cylinder includes a plurality of engaging convexed sections on an outer surface thereof at positions corresponding to the engaging hole sections, the plurality of engaging convexed sections projecting radially outward and located at a prescribed interval; and the fixed-side ring plate and the outer cylinder are engaged with each other by insertion of the engaging convexed sections into the engaging hole sections in the direction of the rotation axis, characterized in that either one of the fixed-side ring plate rotation regulation section and the outer cylinder rotation regulation section is a fitting convexed portion, and the other of the fixed-side ring plate rotation regulation section and the outer cylinder rotation regulation section is a fitting concaved portion; and the fitting convexed portion is in contact with, and thus is fit to, the fitting concaved portion at both of two ends thereof in the circumferential direction. a fixed case including a ring-shaped fixed-side ring plate and an outer cylinder having a cylindrical shape and extending from an outer circumferential edge of the fixed-side ring plate perpendicularly to a planar surface of the fixed-side ring plate; and a rotatable case including a ring-shaped rotatable-side ring plate and an inner cylinder having a cylindrical shape and extending from an inner circumferential edge of the rotatable-side ring plate perpendicularly to a planar surface of the rotatable-side ring plate toward the fixed case; and the fixed case and the rotatable case are fit to each other so as to be rotatable with respect to each other in a clockwise direction and a counterclockwise direction; an accommodation section is defined by the fixed-side ring plate and the outer cylinder of the fixed case and the rotatable-side ring plate and the inner cylinder of the rotatable case; in the accommodation section, flat cables for electrically connecting the fixed case and the rotatable case to each other are accommodated in a wound state; the fixed-side ring plate and the outer cylinder are formed of different members; a fixed-side ring plate rotation regulation section is provided at the outer circumferential edge of the fixed-side ring plate; an outer cylinder rotation regulation section is provided at an outer circumferential edge of the outer cylinder, at a position corresponding to the fixed-side ring plate rotation regulation section; and the fixed-side ring plate includes a plurality of engaging hole sections each having an engaging hole, the plurality of engaging hole sections projecting perpendicularly from the outer circumferential edge toward the outer cylinder and located at a prescribed interval in a circumferential direction; the outer cylinder includes a plurality of engaging convexed sections on an outer surface thereof at positions corresponding to the engaging hole sections, the plurality of engaging convexed sections projecting radially outward and located at a prescribed interval; and the fixed-side ring plate and the outer cylinder are engaged with each other by insertion of the engaging convexed sections into the engaging hole sections in the direction of the rotation axis,