1. An apparatus for controlling a mobile robot having a body, leg locomotion mechanisms for walking operation and work mechanisms for working operation both connected to the body, a first actuator for driving the leg locomotion mechanisms, a second actuator for driving the work mechanisms, a speech input/output unit including a microphone and a speaker, and a controller adapted to control the operation of the leg locomotion mechanisms and work mechanisms by supplying the first and second actuators with a control value which is calculated by multiplying a deviation between a detected value and a desired value by a gain and to control operation of the speech input/output unit to recognize human voice inputted through the microphone and utter a generated response to the recognized human voice from the speaker, wherein: the controller selects one of a silent mode for controlling only the operation of the speech input/output unit, a walking mode for driving the leg locomotion mechanisms to control a walking operation, and a working mode for driving the work mechanisms to control a working operation, and calculates the gain of the control value to be supplied to the first and second actuators corresponding to the selected mode, and wherein the controller calculates the gain at a minimum value when selecting the silent mode, and calculates a smaller gain when selecting the working mode than when selecting the walking mode.