PURPOSE: To obtain the subject string suppliable at a low cost and suitable as a raw material for food, by contracting a continuous material such as a tape made of a collagen material by denaturation.

CONSTITUTION: A continuous material such as tape or film sheet of a collagen material is denaturated to effect the contraction (preferably to 50W97% of the original length) and, as necessary, twisted to obtain the objective string. The denaturation of collagen is preferably carried out by the thermal denaturation induced by passing a continuous collagen material through a hot roll or hot steam atmosphere or by the drying denaturation caused by treating the collagen with an aqueous solution of a collagen denaturation agent and drying the treated collagen. The treatment of the collagen with the aqueous solution of the denaturation agent is preferably carried out by immersing the collagen material in the aqueous solution or spraying the aqueous solution to the collagen.