1. A movement trajectory predicting device predicting a movement trajectory of an object around a vehicle, the device comprising: an object detection unit configured to acquire a position of the object around the vehicle; a speed acquisition unit configured to acquire a ground speed of the object, which is the absolute speed of the object with respect to the ground, by calculating the difference between a current value of the detected position of the object and a position of a fixed object on a road, calculating the difference between a previous value of the detected position of the object and the position of the fixed object on the road, calculating a movement vector from the previous value of the detected position of the object to the current value of the detected position of the object by using the calculated difference between the current value of the detected position of the object and the position of the fixed object on the road and the calculated difference between the previous value of the detected position of the object and the position of the fixed object on the road, and dividing an amount of movement of the movement vector by an update time; a relative movement vector calculation unit configured to calculate a relative movement vector of the object with respect to the vehicle by using speed information on the vehicle and the ground speed of the object acquired by the speed acquisition unit; a classification unit configured to record positions in the movement trajectory of the object acquired by the object detection unit and to classify the recorded positions into a plurality of groups based on the relative movement vector calculated by the relative movement vector calculation unit; and a movement trajectory prediction unit configured to predict the movement trajectory of the object based on a plurality of positions of the object included in at least one of the groups classified by the classification unit.