A fuel injection control device is disclosed that includes a fuel injection valve for performing a fuel injection event at an assumed fuel quantity. The device also includes a rotation detecting device for detecting a change in rotation amount of the output shaft. The device further includes a slip rate detection device for detecting a slip rate between the output shaft and the driven shaft. Also included is an actual fuel injection amount estimating device for estimating an actual fuel injection quantity during the fuel injection event based on the detected change in rotation and the detected slip rate. The device also includes a learning device for learning a deviation based on the difference between the estimated actual fuel injection quantity and the assumed fuel injection quantity. A related method is also disclosed.