Abstract: A serial signal transmission device and a control method for determining polarity which can be used in various systems, for example, air conditioners, and a remote controller. The serial signal transmission device which carries out signal transmission between a plurality of control devices uses two transmission wires with fixed polarity, the serial transmission interface of each control device is equipped with two pairs of differential drivers and receivers, and the driver outputs and receiver inputs of different polarity in these two pairs of drivers and receivers are connected to each other. This device and control method make connection terminals for signal lines on each control device nonpolar, considerably reducing workload required for installing the apparatuses and wiring and eliminating connection errors.