To enhance the cooling efficiency for a shaft body of a screw shaft.

In a screw shaft 2 of a screw conveyor with screw blades 31 mounted on an outer circumferential surface of a shaft body 21, the shaft body 21 is formed of a cylindrical pipe to allow cooling water W to distribute in a hollow part 22 in the cylindrical pipe, and a flow passage area reducing member 4 for reducing the sectional area of a flow passage of cooling water W is installed in the hollow part 22 of the shaft body. The flow rate of cooling water in the hollow part 22 of the shaft body is increased to enhance the cooling efficiency to the shaft body 21 and the screw blades 31.