An ECU as a driving assistance device has a traffic sign detection section, a lane entry detection section and a judgment section. The traffic sign detection section detects traffic signs including a speed limit symbol based on front-image data captured by an in-vehicle camera. The lane entry detection section detects that the own vehicle has entered a ramp lane to leave a highway. The judgment section detects an extent of a current driving lane as the ramp lane when following conditions (c1) and (c2) are satisfied after the lane entry detection section detects the entry of the own vehicle into the ramp lane: (c1) not less than two traffic signs are arranged along the current driving lane in forward direction of the vehicle; and (c2) an arrangement of speed limit symbols included in the detected traffic signs satisfy a predetermined speed reduction pattern.