Headrest device, method of adjusting headrest positiion, and vehicle seat

There is provided a headrest device including: a headrest main body that is provided at an upper end portion of a seat back of a seat in which a passenger sits; a pressure/flexure detecting section that is provided at a vehicle front side of the seat back; a headrest moving section; and a control section that estimates a current back-set, that is a distance between a head portion of a passenger and the headrest main body in a horizontal direction, on the basis of at least one of pressure and flexure detected by the pressure/flexure detecting section and a predetermined relationship between back-set and the at least one of pressure and flexure, and controls the headrest moving section to move the headrest main body such that the back-set becomes a predetermined amount.