To obtain the subject additive capable of surely suppressing the formaldehyde emission from particle boards to a specific level or lower without impairing both the performance and durability of the adhesive used by compounding urea, an acrylic emulsion and water in specified amounts by weight, respectively.

This additive comprises (A) 35-40 pts.wt. of urea, (B) 0.5-5 pts.wt. of an acrylic emulsion, and (C) 50-60 pts.wt. of water; wherein the component A is such one as to reduce free formaldehyde through methylolation or resinification of formaldehyde, and the component B is such one as to compensate the decline in the physical properties of the adhesive used in association with the addition of the component A. Thereby, in the manufacturing process for particle boards using MDI as adhesive, this additive prevents formaldehyde from being absorbed into the MDI layer, does not impair the excellent properties of the particle boards, and enables formaldehyde emission to be surely suppressed to levels of ≤0.5 mg/L.