A decelerating factor estimating device that estimates a decelerating factor of a vehicle, the decelerating factor estimating device comprising: 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 (63, 64, 65, 68) configured to estimate a plurality of decelerating factors based on a relationship among the acquired drive force, the acquired speed, and the acquired acceleration, wherein the decelerating factor estimating unit (63, 64, 65, 68) determines a type of the decelerating factor to be estimated based on a traveling state of the vehicle, and at the time an estimated value of a first decelerating factor is converging, further widens a range of the traveling state of the vehicle to execute an estimation process of a second decelerating factor than a case where the estimated value of the first decelerating factor is not converging, characterised in that: the first decelerating factor is an air resistance coefficient, the second decelerating factor is a vehicle weight, the decelerating factor estimating unit (63, 64, 65, 68) estimates the vehicle weight at the time the vehicle speed is smaller than a threshold value, and at the time the air resistance coefficient is converging, the decelerating factor estimating device makes a speed of the threshold value a higher speed than a case where the air resistance coefficient is not converging. 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 (63, 64, 65, 68) configured to estimate a plurality of decelerating factors based on a relationship among the acquired drive force, the acquired speed, and the acquired acceleration, wherein the decelerating factor estimating unit (63, 64, 65, 68) determines a type of the decelerating factor to be estimated based on a traveling state of the vehicle, and at the time an estimated value of a first decelerating factor is converging, further widens a range of the traveling state of the vehicle to execute an estimation process of a second decelerating factor than a case where the estimated value of the first decelerating factor is not converging, characterised in that: the first decelerating factor is an air resistance coefficient, the second decelerating factor is a vehicle weight, the decelerating factor estimating unit (63, 64, 65, 68) estimates the vehicle weight at the time the vehicle speed is smaller than a threshold value, and at the time the air resistance coefficient is converging, the decelerating factor estimating device makes a speed of the threshold value a higher speed than a case where the air resistance coefficient is not converging. the first decelerating factor is an air resistance coefficient, the second decelerating factor is a vehicle weight, the decelerating factor estimating unit (63, 64, 65, 68) estimates the vehicle weight at the time the vehicle speed is smaller than a threshold value, and at the time the air resistance coefficient is converging, the decelerating factor estimating device makes a speed of the threshold value a higher speed than a case where the air resistance coefficient is not converging.