Investigation of problems of the device structure of a power MOSFET and mass production of it in relation to high breakdown voltage and low ON resistance when an epitaxy trench filling system is employed has revealed that it has the following problem, that is, a high breakdown voltage as expected cannot be achieved because a P-column region does not have an ideal rectangular parallelepipedal shape but has an inverted trapezoidal shape narrower at the bottom thereof and at the same time, has a concentration distribution lower at the bottom. In order to overcome the problem, the present invention provides a semiconductor device including a power MOSFET portion equipped, in an active cell region thereof, a super junction structure formed by a trench filling system, wherein a base epitaxial layer has a multistage structure with the upper portion having a higher impurity concentration.