When a selection signal of a low signal is applied to a first port, a second port becomes either a low or a high signal in accordance with an input signal being a low or a high signal at a third port and the third port can be used as an input port. Further, when a completely synchronized low or high signal is inputted to the first port and the second port, a signal of either a low or a high signal inputted to the second port can be outputted from the third port as it is and the third port can be used as an output port.