In a differential device in which a working window is provided in a case main body of a differential case that can rotate around a first axis, when viewed on the projection plane, orthogonal to the first axis, among joining parts between a flange portion of the differential case and a ring gear, a specific joining part that is the closest to the working window is positioned further outward than an inner end part of the working window in a direction along a third axis orthogonal to the first axis and a pinion axis, and the working window is positioned further outward, in the direction along the third axis, than an imaginary straight line joining the specific joining part and the first axis. Accordingly, rigidity strength of the differential case is enhanced and concentration of stress is prevented from occurring in the working window of the case main body.