A semiconductor device comprising: an isolation region formed extending from a surface of a semiconductor substrate to a first depth position; first and second wells of a first conductivity type formed in said semiconductor substrate; a first transistor formed in said first well, said first transistor including a gate insulating film having a first thickness, source/drain regions of a second conductivity type opposite to the first conductivity type, and a gate electrode; and a second transistor formed in said second well, said second transistor including a gate insulating film having a second thickness thinner than the first thickness, source/drain regions of the second conductivity type, and a gate electrode; wherein said first well has a first impurity concentration distribution superposing a second impurity concentration distribution having a peak at a second depth position deeper than said first depth position, and a third impurity concentration distribution having a peak at a third depth position approximately equal to said first depth position, and said second well has a fourth impurity concentration distribution superposing an impurity concentration distribution being the same as said first impurity concentration distribution of said first well, and a fifth impurity concentration distribution having a peak at a fourth depth position shallower than said third depth position, said fourth impurity concentration distribution having a first peak at said second depth position, a second peak at said fourth depth and a third peak at said third depth position. an isolation region formed extending from a surface of a semiconductor substrate to a first depth position; first and second wells of a first conductivity type formed in said semiconductor substrate; a first transistor formed in said first well, said first transistor including a gate insulating film having a first thickness, source/drain regions of a second conductivity type opposite to the first conductivity type, and a gate electrode; and a second transistor formed in said second well, said second transistor including a gate insulating film having a second thickness thinner than the first thickness, source/drain regions of the second conductivity type, and a gate electrode; wherein said first well has a first impurity concentration distribution superposing a second impurity concentration distribution having a peak at a second depth position deeper than said first depth position, and a third impurity concentration distribution having a peak at a third depth position approximately equal to said first depth position, and said second well has a fourth impurity concentration distribution superposing an impurity concentration distribution being the same as said first impurity concentration distribution of said first well, and a fifth impurity concentration distribution having a peak at a fourth depth position shallower than said third depth position, said fourth impurity concentration distribution having a first peak at said second depth position, a second peak at said fourth depth and a third peak at said third depth position. said first well has a first impurity concentration distribution superposing a second impurity concentration distribution having a peak at a second depth position deeper than said first depth position, and a third impurity concentration distribution having a peak at a third depth position approximately equal to said first depth position, and said second well has a fourth impurity concentration distribution superposing an impurity concentration distribution being the same as said first impurity concentration distribution of said first well, and a fifth impurity concentration distribution having a peak at a fourth depth position shallower than said third depth position, a fourth impurity concentration distribution superposing an impurity concentration distribution being the same as said first impurity concentration distribution of said first well, and a fifth impurity concentration distribution having a peak at a fourth depth position shallower than said third depth position, said fourth impurity concentration distribution having a first peak at said second depth position, a second peak at said fourth depth and a third peak at said third depth position.