The addition of drag-reducing agents to reduce pipe friction loss has attracted attention as a method to conserve energy. In addition to reducing drag, these agents are required to have a low environmental load and conserve natural resources. Therefore, naturally occurring biopolymer additives, which are considered to have a low environmental load, have recently received much attention. Here we focused on nata de coco, a type of biopolymer that exhibits low mechanical degradation, and found that it reduced drag by up to 25% at a concentration of 50 ppm. With respect to the drag reduction (DR) mechanism, we investigated the relation between DR phenomena and the fiber structure of nata de coco by visualization. As a result, we found that the DR effect appeared only when a network of nata de coco fibers was formed in the suspension. In addition, DR increased as the size of the network of nata de coco fibers increased.