A decelerating factor estimating device configured to estimate a decelerating factor of a vehicle includes a drive force acquiring unit configured to acquire a drive force of the vehicle; a vehicle speed acquiring unit configured to acquire a vehicle speed of the vehicle; an acceleration acquiring unit configured to acquire an acceleration of the vehicle; and a decelerating factor estimating unit configured to estimate a plurality of decelerating factors based on a relationship of the acquired drive force, speed, and acceleration, wherein the decelerating factor estimating unit determines a type of the decelerating factor to estimate based on a traveling state of the vehicle.