To easily modify from a single control to a tandem control, by setting a difference-extraction part which subtracts a previous output limit value from an output of the speed control part between a speed control part and either of current control parts in the case of a tandem connection.

In the case of a single control, an output limit value of a limiter of a speed control part 5 is set in a range of +10--10V and a speed control to a dynamometer 1 alone is carried out. In the case of a tandem connection, the output limit value of the limiter of the speed control part 5 is set in a range of +11--11V, and a difference-extraction part 12 is connected between the speed control part 5 and a current control part 6. When an output of the speed control part 5 is +10--10V, the difference-extraction part 12 subtracts 10V or -10V, a control command is not fed to a dynamometer 2, and only the dynamometer 1 is subjected to the speed control, thereby absorbing the power. When the output of the speed control part 5 becomes 11V, 1V is output from a comparator 13 and the dynamometer 2 is subjected to the speed control with a current command value of the 1V.