A field-effect transistor including a gate electrode provided on a first-conductivity-type region of a semiconductor substrate with an insulating film provided between the gate electrode and the first-conductivity-type region, a source region of a second conductivity type provided in the semiconductor substrate on one of sides across the gate electrode, a drain region of the second conductivity type provided in the semiconductor substrate on the other of the sides, the other side facing the one side across the gate electrode, a first region of the first conductivity type provided below the drain region and having a higher concentration than the first-conductivity-type region, a second region of the first conductivity type provided to reach a surface in the semiconductor substrate on the other side and having a higher concentration than the first-conductivity-type region, and an extraction electrode connected to the second region.