PURPOSE: To provide the subject sediment-settling agent composed of a silica sol consisting of plural silica particle groups having different particle size distributions, exhibiting high sediment-settling speed and capable of shortening the time necessary for sediment-settling.

CONSTITUTION: The objective sediment-settling agent is composed of a silica sol consisting of at least two silica colloid particle groups having different particle size distributions satisfying at least one of the formulas D2-n≥1.3D1 wherein D1 is the peak value of the particle size distribution of a silica particle group having the smallest particle size distribution and D2, D3...Dn are peak values of respective particle size distributions of the other one or more kinds of silica colloid particle diameters having larger particle size distributions.