To discharge friction material only in the case of a travel road being a low μ road in a road surface friction coefficient improving device that discharges friction material in front of wheels in the case of the generation of slip quantity exceeding the specified value.

Injection nozzles 10L, 10R are disposed in front of wheels FL, FR. Friction material tanks 16L, 16R are connected to the injection nozzles 10L, 10R through discharge control valves 12L, 12R and pressure pipes 14L, 14R. The friction material tanks 16L, 16R are filled with sand. An ECU 30 opens the discharge control valves 12L, 12R in the case of a travel road being judged to be a low μ road and the execution conditions of ABS control being effected on the basis of the detection signals of an acceleration sensor 32, wheel speed sensors 38FL, 38FR, 38RL, 38RR, and the like.