Relative apparent viscosity η r was measured on suspensions containing neutrally buoyant spherical particles of two different sizes (mean diameter: 398 and 84 μm) with various mixing ratios, using an efflux‐type capillary viscometer. The relative apparent viscosity of the suspension with a single solid component was lowered by mixing particles of different sizes, keeping the total solid volume fraction constant at 0.2. The relative apparent viscosity of the suspension showed a minimum when the volume fraction of the larger particles with respect to the total solids was about 0.6. In the lower shear regime, however, adding a small amount of the smaller particles brought a remarkable decrease in η r of the suspension of larger particles. Through direct observation of the particles flowing through the capillary tube, this effect was attributed to the motion of the larger particles leaving the wall when these particles overtake smaller particles near the wall.