A controller has a plurality of control units that are mutually connected by a communication line and are set at different places of an automobile, each corresponding to a different one of its power windows and serving to control its opening and closing. One of these control units is adapted to transmit a signal, in response to a switch operation, to another of the control units through the communication line to open or close the window corresponding to the latter control unit. When this control unit detects that it has submerged in water, it applies a constant voltage to an interface of the communication line. This prevents communications through the communication line and windows from moving in an unwanted manner.