PURPOSE: To prepare healthy plast of laver applicable effectively for cell fusion, by treating a cultured liquid of microorganisms belonging to Pseudomonas genus to obtain a liquid containing mannan hydrolase and xylan hydrolase, and treating a laver leaf with the liquid.

CONSTITUTION: A microbial strain belonging to Pseudomonas genus and capable of hydrolyzing a hardly digestible polysaccharide, is cultured in a medium containing laver or a laver-originated polysaccharide as an inducing substance, and the cultured liquid is treated to obtain an enzymatic liquid containing at least mannan hydrolase and xylan hydrolase. A laver leaf which may be treated with a protease, is treated with the enzymatic liquid. Optionally, a laver leaf is treated with the enzymatic liquid and a protease.