The starting material mixture containing gelatin thus obtained is then heated to dissolve the components. The heating temperature may be the dissolving temperature for gelatin or higher but is preferably in the range from 30 range, the viscosity is increased excessively to impair the spinning characteristic, while a temperature higher than this range may also bring about deterioration of the gelatin, whereby the spinning characteristic will be impaired. For the sake of the viscosity after heating and the strength of the fibres formed, a temperature around 50 preferred.