The effect of ethylated soy protein-based bioflocculant (EtSP) as a filter aid reagent was investigated. The efficiency of EtSP as a filter aid was evaluated in terms of the specific cake resistance, α, and was compared with chitosan and polyaluminum chloride (PAC). Diatomite and kaolin were used as model particles. Total filtration resistance, R, decreased with increasing flocculant dosage (wt.%, flocculant/particle) and was almost constant in the range of 1 wt.% or more for both particles. The α value was significantly decreased from 1.01 × 1011 to 9.01 × 1010 m/kg for diatomite and from 5.11 × 1010 to 5.20 × 109 m/kg for kaolin by the addition of EtSP in the case of 1.0 wt.%. The α value for cakes formed by EtSP was much smaller than that formed by chitosan and PAC. In the case of diatomite, in the dose range of 0.5–1.0 wt.%, the α value for cakes formed by EtSP and chitosan was almost the same. However, at the excess dose of 2.0 wt.% over, the α value formed by chitosan abruptly increased. In the case of kaolin, in the dose range of 1.0–2.0 wt.%, the α values of chitosan and PAC were mostly the same, however, these values were larger by ca. nine times than that of EtSP.