An apparatus for forming spiral steel pipe by spirally curling a strip of metal with a row of interior forming rolls set on an interior forming stand and rows of exterior forming rolls respectively set on exterior forming stands. A mount is provided which extends in the direction of axis of the interior forming stand in such a manner as to cover the interior forming stand wholly except the part thereof equipped with the interior forming rolls; and a plurality of rows of interior supporting rolls are disposed on the outer surface of the mount nearly in parallel with the edges of the spiralled strip.