Abstract: There is disclosed a mechanism for stationarily supporting an operation panel mounted on a steeering wheel of a movable body. This mechanism includes two external gears between which said steering wheel is interposed, and two satellite gears resiliently urged toward the external gears. One of the external gears is stationary and the other has an operation panel secured thereto. Two satellite gears are rotated in the same direction and as a result, the operation panel is rotated in the opposite direction to that of the steering wheel and keeps substantially stationary. Further, there is disclosed a signal transmitting system for a radiotelephone in the movable body. A microphone is provided on the operation panel and a speaker unit is located at any position in the movable body. A duplex signal transmitting path having a howling control function is provided between the microphone and the speaker unit.