Abstract: A semiconductor device having high breakdown withstand voltage includes a first element which is a normally-on type transistor made of nitride compound semiconductor, a second element which is connected to the first element in series and is a transistor having withstand voltage between a source and a drain lower than withstand voltage of the first element, a first diode which is connected between a gate of the first element or a gate of the second element and a drain of the first element so that a cathode of the first diode is connected at the drain's side and has predetermined avalanche withstand voltage, and a first resistance connected to the gate to which the first diode is connected. The avalanche withstand voltage of the first diode is lower than breakdown voltage of the first element.