To provide a method of manufacturing a ribbed spiral steel tube by which the generation of cracks in the root part of a rib in a forming stage can be prevented without increasing its production cost.

In the method of manufacturing the ribbed spiral steel tube 2, a hot-rolled steel sheet 1 is preliminarily heated before applying bending work by two-point supporting and one-point concentrated load with forming rolls 3, 4, 5. Preferably, the steel sheet is beforehand heated not higher than 200C in this method of manufacturing the ribbed spiral steel tube.