A semiconductor device of an embodiment includes a first diode having a first anode and a first cathode, the first anode connected to either one of first and second electrodes of a first transistor having the first and second electrodes and a first gate electrode; a first electric resistor having a first one end connected to the first cathode and a first other end connected to positive pole of a direct-current power source; a first capacitor having a second one end and a second other end connected to the first cathode; a second capacitor having a third one end connected to negative pole of the direct-current power source and a third other end connected to the second one end of the first capacitor; and a second switching element connected in parallel to the second capacitor.