PURPOSE: To prevent wheels from reslipping by detecting and storing a limit frictional force between the wheels and rails at slip starting point, and using a value slightly smaller than this value as a reset command value of tensile force of the periphery of the wheel.

CONSTITUTION: When a slip occurs, a switch 14 is closed by the output of a slip detector 5, and a slip acceleration Vs and a motor current I of a slip shaft at the slip starting time are respectively stored in a slip acceleration memory 7 and a current memory 9. A current IuSW corresponding to the limit frictional force is calculated on the basis of the acceleration Vs and the current I, a current reduction command value Ip2 and a prescribed command value ΔI are added to the current IuSW, and stored in a memory 13. When the slip is detected, the value Ip2 is output, the value stored in the memory 13 is input to an adder 16 at the suitable time of the process that the slip speed reduces, and added to the value Ip2 at that time.