In a semiconductor device having a high voltage transistor, a first well region of the high voltage transistor is formed in a semiconductor substrate as a channel region. The first well region has a first conductive type. Second well regions of the high voltage transistor are formed in the semiconductor substrate as a source region and a drain region to sandwich the first well region. The second well region has a second conductive type. A surface of the first region and surfaces of the second well regions have a flat plane.