To provide a headlight device, in which a change of the region of illumination afforded by the headlamp can be initiated substantially punctual to the start of cornering of the motorcycle, the headlight device includes a headlamp for illuminating forwardly of the motorcycle, a light-distribution adjusting mechanism for changing a region of illumination, a bank angle detecting unit for detecting a bank angle δ of the motorcycle in reference to an angular velocity ωy about a vertical axis of the motorcycle, an angular velocity ωr about a longitudinal axis of the motorcycle and a motorcycle velocity v, and a light-distribution controller for controlling the light-distribution adjusting mechanism based on the detected bank angle δ to change the region of illumination into an area further away from and at least inwardly of a direction of turn of the motorcycle during a cornering.