The purposes of this study were to examine applicability of waste egg shells as conditioning agent, to determine the optimum conditions for improving digested sludge dewaterability and to compare with before and after adding them. It estimated that the optimum conditions were dosage 30 g/L, particle size 10mesh, temp. $30^{circ}C$, pH 7 and dewatering pressure $2.66{pm}10^4{;}N/m^2$. On the basis of induced optimum conditions, if adding waste egg shells in digested sludge, its specific resistance was $1.13{pm}10^{12}{;}m/kg$ and was low about 3 times than raw digested sludge $3.5{pm}10^{12}{;}m/kg $. Therefore, in view of waste recycling, it seemed that we could utilize waste egg shells as conditioning assent for improving dewaterability of digested sludge.