A belt meandering preventing device 1 for a running test device 2 is provided with a first control means 14, a second control means 15, and a third control means. The first control means 14 detects the position of a belt 6 and controls a drum rocking means 8 so that the difference between the detected position of the belt 6 and a predetermined target position is zero. The second control means 14 detects the amount of rocking of a driven drum 5 rocked by the drum rocking means 8 and controls the drum rocking means 8 so that the detected amount of rocking of the driven drum 5 is a target amount of rocking.