An alert device alerts a driver sitting on a driver's seat in a vehicle by winding a seat belt on the driver's seat with a retractor. The alert device includes a seat position information acquisition unit configured to acquire seat position information relating to a position of the driver's seat, a height estimation unit configured to calculate art estimated height that is an estimated value of the driver's height based on the seat position information, a winding strength setting unit configured to set a winding strength that is a strength for winding the seat belt with the retractor based on the estimated height, and a retractor control unit configured to operate the retractor to wind the seat belt with the winding strength.