To detect and stop slips of a traveling carriage via a simple structure.

Rotary encoders 28 and 29 detect the speed of rotation of the driving and driven wheels of a traveling carriage, and a slip detecting means 36 detects the presence or absence of slips from the speed difference, and the slippage if any. Upon slip detection, accelerating/decelerating conditions and braking conditions are changed through accordingly changed readout positions of an accelerating/decelerating condition table 46 specifying acceleration/ deceleration and start points of accelerating/decelerating and of a braking condition table 48 specifying start points of braking.