In order to improve the sonochemical efficiency of a sonoreactor, a metallic reflection plate is set in liquid. With the placement position and the diameter of the reflection plate varied at the same liquid height, a decolorization experiment using iodine and disodium hydrogenphosphate in starch solution has been carried out in order to examine the effects of the reflection plate on ultrasonic reaction. With the placement position of the reflection plate close to the liquid surface, the range of reaction field becomes much larger and the reaction proceeds more quickly. The diameter of the reflection plate has an optimum value for the sonochemical efficiency of the sonoreactor.