PURPOSE: To make the cooking time to achieve a desired degree of delignification readily determinable by analyzing a monomeric compound produced by lignin degradation during the cooking.

CONSTITUTION: A monomeric compound produced by lignin degradation is analyzed during the cooking in an alkali cooking process for wood or other cellulose-containing materials, especially a kraft and an anthraquinone cooking processes and the cooking time for enabling a desired degree of delignification to be achieved is determined on the basis of the related concentration thereof. A related composition of a monomeric lignin derivative-containing fraction is preferably analyzed by any of a high-performance liquid chromatography or a gas chromatography.