A flywheel device includes a mass-side rotor including a mass ring and a pair of holder plates connected to both sides of the mass ring. The mass-side rotor is connected to a crankshaft with a drive plate therebetween. Between the mass-side rotor and the drive plate, a positioning apparatus for positioning those coaxially with each other is provided. The positioning apparatus includes: an annular positioning protrusion which is press formed in at least one of the pair of holder plates to project laterally outward therefrom, and which is disposed coaxially with the mass-side rotor; and a positioning recessed portion which is press formed in the drive plate coaxially with the crankshaft, and which is fitted to the positioning protrusion. Accordingly, it is possible to enable coaxial positioning of the mass-side rotor and the drive plate without performing burdensome machining on the mass ring for forming a positioning apparatus.