To provide a corrugating medium for a high-quality corrugated board which is especially excellent in water resistance and strength and producible over a long time without stopping a paper machine immediately after the papermaking process, and suppresses the generation of insoluble waste stuff to prevent the accumulation of the stain in the process.

The corrugating medium is formed of a raw pulp obtained from a defibrated pulp defibrated in conformity to JIS P 8220 by classifying or beating the pulp in a manner to attain a Canadian standard freeness of 180-350 ml in conformity to JIS P 8121, a functional treating agent at least containing a waterproofing agent is applied to at least one surface of the base paper during the drying step and before and after the drying step, and the corrugating medium has a Stoeckigt sizing degree of 0.001-0.05 sec/(g/m2) per unit basis weight in conformity to JIS P 8122 and a Dennison wax value of 7A or over in conformity to JIS P 8129.