Cocoons were dried under low-temperature (60°C or lower) conditions, rather than under conditions where the temperature is higher than 100°C which have been conventionally employed when producing raw silk from cocoons. Further, cocoon boiling was carried out by performing a vacuum infiltration treatment at a low temperature (a reduced pressure treatment), rather than a high-temperature treatment at 100°C or higher, or a treatment with near-boiling-point hot water which was conventionally employed. As an alternate cocoon boiling method, a combination treatment involving the swelling of sericin with an alkali and the penetration of warm water into a cocoon layer by using a surfactant, an enzyme, or such was carried out. Furthermore, reeling was carried out at a temperature lower than that conventionally used. As a result, it was found that raw silk can be produced from cocoons of silkworms that express a foreign gene without impairing the properties of the foreign protein contained in the cocoons of the foreign gene-expressing silkworms.
