PURPOSE: To make it possible to carry out gas cutting in a short time and to reduce the unit consumption of oxygen by recognizing the cutting length of a round steel material corresponding to the swing angle of a blow-pipe and cutting with the velocity in the reverse proportion with the cutting length.

CONSTITUTION: The swing angle θ from the reference position of the blowpipe 23 is detected with an angle detector 1 arranged at the rotating shaft of a geared motor 22, and the signal is transmitted to a cutting length computing element 2. In the computing element 2, the cutting length is made to be a line segment ab=L between the interdections (a) and (b) with the straight line A drawn in the longitudinal direction of the blowpipe 23 and the circle B showing the outline of the round steel material 21 and calculated as the preinputting calculating equation. The signal of this cutting length L is transmitted to a swing velocity computing element 3 and the swing velocity V of the blowpipe 23 is calculated with the following equation. V=K (1/L), (wherein, K: constant). The command is transmitted from the computing element 3 to a swing velocity controller 4 and the geared motor 22 is driven to make the necessary swing velocity. Number or revolutions of the geared motor 22 is detected with a tachometer 5 and inputted to the swing velocity controller 4 to control the velocity.