In a vehicle driving assist system, an external environment recognition unit detects a 3-D obstacle inhibiting the driving direction of the vehicle based on an image captured by a stereo camera and determines whether the image ahead of the vehicle is in a condition that makes visual perception of the 3-D obstacle difficult. A controller calculates a required alarm working distance between the vehicle and the 3D obstacle, at which an alarm unit is worked when the vehicle approaches the 3D obstacle, and a required brake operating distance, at which a brake is operated by a brake applying unit, based on a vehicle traveling speed. If it is determined that an external environment is in a condition that makes visual perception difficult, the required working and operating distances are extended by predetermined values to set the working and operating timings earlier.