PURPOSE: To facilitate the discrimination of the limit to the slippage rate by generating an alarm when the slippage rate exceeds a specified value as calculated based on the output of a sensor.

CONSTITUTION: Signals from a first sensor 1 for generating the number of pulses proportional to the revolutions of the left drive wheel and a second sensor 2 for generating the number of pulses proportional to the revolutions of the right drive wheel are supplied to a control section 4 to calculate the slippage rate and the results of the calculation are indicated on a display 5. When a switch 7 selects the indication of the revolutions, the display 5 indicates the revolutions of right and left drive wheels. When the slippage rate calculated is larger than the set value by a slippage rate setting section 3, a buzzer 6 is driven.