PURPOSE: To improve the yield, efficiency and quality of rolling in a method of forming triangular hollow slits on both sides edges of flat ingots with plural split calibers provided at the center in the bottom part, by specifying the shape of the split calibers.

CONSTITUTION: In order to produce a rough shape ingot for H-steel, a flat ingot obtained by continuous casting or the like is heated, and triangular hollow slits are formed on both side edges thereof. In this formation, the ingot with the width direction thereof held top and bottom is roughly rolled with the split calibersIWIII formed to the rolls of duplex type reversible rolling mills. The calibersIWIII have crest parts 1 of a prescribe height (h) and a vertical angle θ, which vertical angle θ is made constant and which height (h) is so set as to increase gradually. The ingot having the slits is force expanded in the slit parts with the box calibers IV formed to said rolls and the slit parts are flattened by plural times of passes. The width direction of the ingot is positioned horizontally, and is rolled with the forming calibers V provided to said rolls in order to reduce the thickness in the web part and to trim the entire sectional shape.