To enhance the fixing rigidity of a flywheel and avoid the size enlargement of a device, in a power transmission device having the flywheel and a damper unit.SOLUTION: A device comprises a flywheel 2 having a plurality of fixing holes 5, and a damper unit 20. The damper unit 20 has an input-side plate 21, a hub flange 22 and a pair of coil springs 47. The input-side plate 21 and the hub flange 22 have a plurality of assembling holes 26, 43 which are arranged at positions corresponding to the fixing holes 5 of the flywheel 2. The plurality of assembling holes 26, 43 are aligned in a circumferential direction and arranged so that a pair of accommodation spaces C wider than the other portion in interval between the adjacent assembling holes are formed. A first window part 21a and a first window part 41a for accommodating a pair of the coil springs 47 are formed outside the accommodation spaces C in a radial direction.SELECTED DRAWING: Figure 2