A single facer which includes a rotating corrugating roll for corrugating a medium; an endless belt for pressing a liner against the medium on the corrugating roll; at least a pair of belt rolls for running the endless belt; and a direction adjusting mechanism for changing the running direction of the endless belt with respect to the rotating shaft of the corrugating roll within the running plane of the endless belt to correct zigzag or sideway motion of the belt occurring while the belt is running; a detector for detecting sideway displacements of the endless belt; a computer for computing a positional correction to be made in the width direction based on a signal from the detector; and a controller for controlling the direction adjusting mechanism based on a value computed by the computer. In addition to the adjustment of the running direction of the endless belt, a tension balance between the opposite edges of the endless belt can be corrected by adjusting the relative angle between the shafts of the belt rolls.