In order to enhance the decomposition performance of a small sonoreactor, a metallic reflection plate was placed in a liquid. The material, placement position and diameter of the reflection plate were changed. The reaction field was observed to be produced below the reflection plate, and became stronger with increasing reflectivity of the reflection plate. The placement position and diameter of the reflection plate had the optimal values for the decomposition performance, and the decomposition performance was enhanced by about 20% at the optimum condition.