PURPOSE: To reduce the work-hardening in the edges of a slit steel belt by heating the steel belt upstream of a slitter blade so as to slit the belt of predetermined temperature.

CONSTITUTION: Steel belt 3 drawn out from a steel belt coil 2 is advanced through a leveller 4 for correcting the curvature remaining in the belt and a thickness measuring device 5 to enter into a heater 6, in which the steel belt 3 is heated so as to be slit by a slitter blade 7 with a temperature of the belt from 50°C to 200°C. The belt 3, after slit by the slitter blade 7, is passed through a loop 9 for shaping and a pad 10 for pressing, to be finally wound around a ten sion reel 12. This constitution enables the work-hardening in the edges of the slit steel belt to be reduced.