To autonomously detect bent parts of rails and control speed by a simple constitution by providing sensors for detecting the rails at the front of a truck.

At the front of a truck man body 3, a pair of sensors 4a, 4b are mounted at locations opposed to rails 1 with the same space as that of the rails 1. The sensors 4a, 4b detect the rails 1 by irradiating the rails 1 with light and receiving light reflected by the rail 1. While a truck 2 is traveling on linear parts of the rails 1, both of the sensors 4a, 4b become 'ON' states that the sensors 4a, 4b detect the rails 1. The speed of the truck 2 is controlled by judging when the right sensor 4b becomes 'OFF' in which the rails 1 can not be detected earlier as a right curve and judging when the left sensor 4a becomes 'OFF' earlier as a left curve because the bend radius of the inner rail 1 is smaller than that of the outer rail 1.