In the field of civil engineering, the use of superplasticizer is expected to be applied practically to improve workability of low slump concrete. It is supposed that low slump concrete get smaller effect of superplasticizer or larger tendency of segregation because of its larger size of aggregate, smaller s/a or smaller unit cement content. But studies on the use of superplasticizer to low slump concrete are not enough, and especially reports about superplasticized concrete using aggregate of more than 40mm size is scarce.From such viewpoints, this study was aimed to clarify the influence of unit cement content on the effect of superplasticizer by examining the properties of superplasticized concrete in which slump of base concrete was 4 or 8cm and cement content was varied.The essential conclusions obtained are as follows.(1) The effect of superplasticizer on consistency of concrete changed with unit cement content (or unit fine powder content) of base concrete, and the consistency increased with increasing cement content.(2) There existed the minimum content of superplasticizer required to obtain its effect according to unit cement content, and it remarkably changed at about 260kg/m3 of unit cement content (or 390kg/m3 of fine powder content).(3) There existed the maximum possible gain in slump obtained from the addition of superplasticizer, which changed with unit cement content of base concrete. Segregation was produced if slump higher than the maximum value was attemped without increasing fine powder content of concrete.(4) The properties of superplasticized concrete of low slump except the above mentioned were almost the same as that of high slump.