PURPOSE: To improve confortability during the change in the command speed while enabling optimal control at the restart of the main controller due to coasting by providing a shunt field current controlling device and air brake with the output of a preference circuit.

CONSTITUTION: The constant speed operation apparatus is provided with a command value transmitting section. The section is equipped with a comparator 1 for receiving a command speed So and an actual speed S as input, a limiter circuit 2 for regulating and producing a differential output, a command value sudden change suppressing circuit for suppressing sudden output change against any sudden input change, an input lowering circuit which lowers the input of the circuit 3 to zero when fed with no command speed So while applies the output of the limmiter circuit 2 to the suppressing circuit 3 as it is when with a command speed So and a preference circuit 5 which produces the output equivalent to the maximum command brake force of the limiter circuit 2 when fed with no command speed So while produces the output of suppressing circuit 3 in preference when with a command speed So.