The effectiveness of two types of procedure for removing large air bubbles in order to increase the percentage of small air bubbles, which is envisaged to be effective for improving the self-compacting properties of fresh concrete, by the addition of an antifoaming agent during mixing has been investigated. In the method of removal using an antifoaming agent after entraining many small air bubbles by divided water mixing, in all the cases investigated, namely adding antifoaming agent at the same time as the primary water, adding antifoaming agent at the same time as the secondary water, and adding antifoaming agent finally, it was not possible to remove large diameter air bubbles, on the contrary they increased. On the other hand, in the method of removing by mixing all together and adding the antifoaming agent finally after a large quantity of air bubbles has been entrained, if the quantity of air entraining agent is increased, the percentage of small diameter bubbles is increased, and the percentage of large diameter bubbles can be reduced.