Provided is a fin-type transistor having an oxide semiconductor in a channel formation region in which the channel formation region comprising an oxide semiconductor is three-dimensionally structured and a gate electrode is arranged to extend over the channel formation region. Specifically, the fin-type transistor comprises: an insulator protruding from a substrate plane; an oxide semiconductor film extending beyond the insulator; a gate insulating film over the oxide semiconductor film; and a gate electrode over and extending beyond the oxide semiconductor film. This structure allows the expansion of the width of the channel formation region, which enables the miniaturization and high integration of a semiconductor device having the transistor. Additionally, the extremely small off-state current of the transistor contributes to the formation of a semiconductor device with significantly reduced power consumption.