Control knob which operates multiple systems

A control knob capable of operating multiple systems has a first rotational body and a second rotational body coaxially arranged, and a shaft connected to the second rotational body. Opposite the shaft, a movable body is moved axially in response to rotation of the first rotational body. Engaging units have concaves or convexes, for example, similar to teeth. The engaging units are provided on the shaft or the movable body. An engaged unit is elastically biased toward one of the engaging units, and engages the concaves or convexes. In use, the control knob may be configured to permit selection of a function by rotation of the first rotational body, and to permit adjustment of the function by rotation of the second rotational body.