An isolator circuit capable of two-way electrical disconnection and a semiconductor device including the isolator circuit are provided. A data holding portion is provided in an isolator circuit without the need for additional provision of a data holding portion outside the isolator circuit, and data which is to be input to a logic circuit that is in an off state at this moment is stored in the data holding portion. The data holding portion may be formed using a transistor with small off-state current and a buffer. The buffer can include an inverter circuit and a clocked inverter circuit.