The invention relates to a semiconductor device and a manufacturing method thereof. An object of the present invention is to provide stable withstand voltage characteristics, reduce turn-off losses along with a reduction in leakage current when the device is off, improve controllability of turn-off operations, and improve blocking capability at turn-off. An N buffer layer includes a first buffer layer joined to an active layer and having one peak in impurity concentration, and a second buffer layer joined to the first buffer layer and an N- drift layer, having at least one peak point in impurity concentration, and having a lower maximum impurity concentration than the first buffer layer. The impurity concentration at the peak point of the first buffer layer is higher than the impurity concentration of the N- drift layer, and the impurity concentration of the second buffer layer is higher than the impurity concentration of the N- drift layer in the entire area of the second buffer layer.