PURPOSE: To prepare the subject agent for preventing plugging, capable of preventing plugging due to scale separation and sticking and also capable of preventing corrosion, abrasion or the like of a suction roll by including malic acid as an effective principal ingredient.

CONSTITUTION: The objective agent for preventing plugging includes malic acid as an effective principal ingredient. The prevention of plugging is preferably carried out by pouring shower water for sealing, added with 10-500 ppm malic acid, onto the inner side of a suction roll shell.