A semiconductor device with favorable electrical characteristics is to be provided. A highly reliable semiconductor device is to be provided. A semiconductor device with lower power consumption is to be provided. The semiconductor device includes a gate electrode, a first insulating layer over the gate electrode, a metal oxide layer over the first insulating layer, a pair of electrodes over the metal oxide layer, and a second insulating layer over the pair of electrodes. The first insulating layer includes a first region and a second region. The first region has a region being in contact with the metal oxide layer and containing more oxygen than the second region. The second region has a region containing more nitrogen than the first region. The metal oxide layer has at least a concentration gradient of oxygen in a thickness direction, and the concentration gradient becomes high on a first region side and on a second region side.