To provide thinnings which can be modified by making good use of lignin.

The method for processing thinnings comprises the steps of putting thinnings sawn up in a determined shape into a treatment tank of hermetically closed structure, exhausting/removing air and moisture included in the wood tissue by putting the thinnings once under reduced pressure, then pouring a treatment solution containing 15-50% lignin to make it penetrate/impregnate into the wood, where the treatment solution is obtained from a waste solution of a papermaking process, or from a lignin product comprising mainly of a lignin sulfonate separated from the waste solution or a lignin product obtained by subjecting the lignin sulfonate product to an alkaline chemical treatment under high temperature/high pressure to effect desulfonation, demethylation, oxidation reaction, etc., further pressurizing the reaction system by using a pressure pump to accelerate permeation of the treatment solution into the wood, then taking out the treated wood from the treatment tank and drying it in air at a normal temperature to fix the lignin in the wood, and thereafter performing heat treatment to oxidize/heat-set to fix the lignin component.