PURPOSE: To obtain an assistant for forming paper sheets, consisting of water- swellable fibers containing combined carboxyl groups in a specific amount and capable of improving dispersibility, adhesive properties to mutual fibers or fibers to substrates, capturing properties of the substrates, etc.

CONSTITUTION: The above-mentioned assistant for forming paper sheets is obtained by bonding and containing carboxyl groups in an amount of ≥0.4mmol/ g, preferably 0.5W5mmol/g in water-swellable fibers having ≥1g/d, preferably ≥1.5g/d wet strength. The afore-mentioned fibers are desirably short fibers cut to a length within the range of 0.5W50mm, preferably 1W20mm collectively considering dispersibility, reinforcing effects, etc. The water swellability is desirable about ≥3 times, preferably 5W300 times based on the own weight of the fibers. Although the assistant for forming paper sheets of this invention is used in an amount suitably set according to the field of application, the amount is desirably within the range of about 0.1W15%, preferably 0.5W10% on the basis of the matrix weight.