A semiconductor device is provided, in which work of a parasitic bipolar transistor can be suppressed and a potential difference can be provided between a source region and a back gate region. A high voltage tolerant transistor formed over a semiconductor substrate includes: a well region of a first conductivity type; a first impurity region as the source region; and a second impurity region as a drain region. The semiconductor device further includes a third impurity region and a gate electrode for isolation. The third impurity region is formed, in planar view, between a pair of the first impurity regions, and from which a potential of the well region is extracted. The gate electrode for isolation is formed over the main surface between the first impurity region and the third impurity region.