The first invention aims to provide a method for preparing stiffer bulky pulp at a freeness level comparable to that obtained by mechanical treatments to improve the bulkiness of product paper by searching for a method capable of adjusting freeness while suppressing damage to pulp fibers and internal fibrillation in a process for producing pulp. The second invention aims to strip contaminants deposited on pulp fibers and/or inorganic particles to reduce adhesion of the contaminants and decrease microorganisms in a treatment of process water containing long pulp fiber fractions used in a process for producing pulp and paper, thereby clarifying process water and thus preparing high-quality pulp and paper.According to the first invention, cavitation is generated in a pulp suspension preferably by a liquid jet and the resulting fine bubbles are introduced into the pulp suspension to adjust the pulp suspension to a desired freeness by means of the impact force induced by collapse of the fine bubbles in a process for producing pulp. According to the second invention, process water containing 50% by weight or less, based on total solids, of long pulp fiber fractions used in a process for producing pulp and/or paper is treated by cavitation generated by a liquid jet and then returned to the process for producing pulp and/or paper and reused.