An electro-hydraulic governer employing a duplex digital controller system for the optimum speed control of a steam turbine running with increasing speed in the starting stage or under load. Each of the digital controllers is operable with both a control mode and a stand-by mode, and when one of the digital controllers is placed in the control mode for controlling the rotating speed of the turbine, the other is placed in the stand-by mode to prepare for possible failure of the controller operating with the control mode. A failure detecting unit detects failure of the digital controller operating with the control mode. An output switching unit disconnects the faulty digital controller from the control system and switches over the operating mode of the stand-by digital controller from the stand-by mode to the control mode for the continuous control of the rotating speed of the turbine. A display is provided on an operator's console to display the outputs of these two digital controllers, and the faulty digital controller is detected by non-coincidence of the displayed information. The faulty digital controller having been disconnected from the control system by the output switching unit is repaired as quickly as possible, and after necessary repair, it stands by to prepare for possible failure of the other controller operating with the control mode.