1. A collision avoidance assistance device configured to execute collision avoidance assistance for a vehicle with respect to an object around the vehicle, the collision avoidance assistance device comprising: an object detection unit configured to detect the object; a planned execution timing determination unit configured to determine a planned execution timing at which the collision avoidance assistance is planned to be executed, based on the result of detection by the object detection unit; a motion characteristics value acquisition unit configured to acquire a motion characteristics value obtainable from at least a jerk of the vehicle in a comparison target time span which is a predetermined time span later than a detection timing at which the object detection unit detects the object and prior to the planned execution timing; and a collision avoidance assistance determination unit configured to determine whether or not to execute the collision avoidance assistance at the timing prior to the planned execution timing based on the result of comparison between the motion characteristics value acquired by the motion characteristics value acquisition unit and a reference motion characteristics value in the comparison target time span obtainable from a history of the past motion characteristics values.