A hydraulic power transmission device includes a front cover, a torque converter main body, a lock-up device, and a dynamic damper. The dynamic damper has a base plate, inertia rings and lid members that configure an inertial body, viscosity attenuation portions, and an elastic coupling portion. The base plate is fixed to an output plate. The inertial body can move in the rotation direction relative to the base plate. The viscosity attenuation portions can generate a variable hysteresis torque according to the relative speed difference between the base plate and the inertial body.