The fluid damper device includes a rotor inserted into a case, and a cover fixed to an opening portion of the case. A thin portion is formed at an end portion of the case on one side in an axis direction, and a contact portion which is in contact with a small-diameter portion of the cover in the axis direction is formed at four positions of an inner circumferential surface of the thin portion at equal angular intervals. A partitioning convex portion which partitions a damper chamber in a circumferential direction is formed on an inner circumferential surface of the case, and the contact portions are formed at positions corresponding to the partitioning convex portion in the circumferential direction. A concave portion recessed radially outward is formed at a position different from the contact portion in the circumferential direction, and a welding convex portion is formed in the concave portion.