PURPOSE: To provide a reinforcing agent for corrugated board, which is suitable for use in coating a core and a liner in a corrugater and does not contain any solvents so that it is very efficient and economical, by mixing an emulsion of low-molecular polyethylene and water with an aq. sodium silicate soln.

CONSTITUTION: An emulsion of polyethylene having an MW of 500W5,000 and water are blended with an aq. soln. of sodium silicate contg. 20W38% silicon dioxide and 7W20% sodium oxide (e.g. JIS Standard No.1 or No.2 sodium silicate) in a ratio of sodium silicate to polyethylene of 80W98 to 20W2 on a solid basis and in such a proportion as to give an aq. dispersion having a solid content of 20W50%. The mixture is stirred to mix them each other, thus obtaining a reinforcing agent for corrugated board. This reinforcing agent can be used as such without reconstructing the existing corrugater, and corrugated board having excellent strength can be produced without causing bonding troubles between base paper and heating rollers.