PURPOSE: To increase fatigue strength of a rear axle housing by removing unevenness due to mold deviation, burrs, traces of welding correction, traces of grinding etc. caused in manufacturing a rear axle housing made of cast steel by blasting work.

CONSTITUTION: After casting a rear axle housing as a cast steel product M, the gate 2 is ground by a grinder 3 or acute angled part 2a is ground by welding correction. Then, blasting work by shot peening is performed on the whole body of the rear axle housing 1. Shot peening is performed by blasting about 0.3W4mm steel shots at blasting speed of 10W50m/sec. Compressive stress is caused on the surface of the rear axle housing by spot peening and internal tensile stress is cancelled by this and concentration of stress is prevented. Consequently, fatigue strength is increased.