A normally-off vertical nitride semiconductor transistor device with low threshold voltage variation includes a drift layer containing a nitride semiconductor, a channel region electrically connected to the drift layer, a source electrode, a drain electrode, a gate insulating film, and a gate electrode. The gate insulating film includes at least a first insulating film located at the channel region side, a second insulating film located at the gate electrode side, and a third insulating film between the second insulating film and the gate electrode, wherein the second insulating film has charge traps with energy levels located inside the band gaps of both the first and third insulating films, and the threshold voltage is adjusted by charges accumulated in the charge traps. The threshold voltage is used to block flowing current by substantially eliminating conduction carriers of the channel region by voltage applied to the gate electrode.