A damper device comprises: a first shaft that outputs rotational power from a power source; a second shaft that transmits rotational power to a gear mechanism and has outer splines; a first rotational member(s) to which the rotational power of the first shaft is transmitted; a second rotational member spline-engaged with the outer splines; a damper unit that absorbs torque fluctuation between the first and second rotational members; and an inertial body having inner splines spline-engaged with the outer splines and having an annular portion. The tooth parts of the inner splines are pressed into contact with the tooth parts of the outer splines in the circumferential direction of the inertial body.