In one embodiment, a multiplex control device includes three or more control modules to execute same operations for same input signals, and a majority decision module to output an output signal that matches majority of output signals outputted by the control modules. Each control module includes an input module to convert an input signal into an input value, a first determination module to obtain input values from input modules of respective control modules to determine whether majority of input values among the obtained input values match, an operation executing module to execute an operation using the matched input value to generate an output value, a second determination module to obtain output values from operation executing modules of respective control modules to determine whether majority of output values among the obtained output values match, and an output module to convert the matched output value to generate an output signal.