A phosphate rock is one of the natural resources of which exhaustion have been recognized in the world. Incinerator ash of sewage sludge is a hopeful substitute resource of phosphate rock. In this study, the increase in phosphorus recovery efficiency in the alkaline elution of incinerator ash of sewage sludge was investigated by adding coagulants to the sludge transported with pressure. PAC, sodium aluminate and poly-iron sulfate were used as coagulants. As a result, coagulant restrained phosphorus discharged from sludge by pH adjustment and decreased phosphorus in supernatant. PAC has wide range of use optimum pH and phosphorus recovery efficiency by alkali solution increased from 49% to 84% at pH4.