Impinging atomization, which has been widely utilized in liquid rocket propulsion systems, is able to produce fine drops at a rated operation. In contrast, the atomization characteristics deteriorate under off design conditions when injection velocity comes to be slower. In the present study, for improving atomization characteristics at off design conditions, an effective technique is verified utilizing small amount of gas (microjet) injection. The microjet is supplied from a pressurized reservoir and is injected from the center of the liquid nozzles toward the impingement point. To clarify the flow field and the mechanism of the effect, experimental visualizations, drop size measurements and corresponding numerical analyses are carried out. It is elucidated that Sauter Mean Diameter (SMD) becomes one-tenth of the original SMD by the microjet injection with the amount of only 1% of liquid mass flow rate. The dominant non-dimensional number is found to be the ratio of the dynamic pressure (microjet/liquid jet) at the impingement point. The optimized atomization efficiency is achieved when the dynamic pressure ratio is approximately two.