In a method of manufacturing a high withstanding voltage MOSFET, a region to be doped with impurities and a region to be doped with no impurity are provided when ion implantation of the impurities is performed in the channel forming region, for controlling a threshold voltage. The region to be doped with no impurity is suitably patterned so that impurity concentration of the channel forming region near boundaries between a well region and a source region and between the well region and a drain region having the same conductivity type as the well region may be increased, to thereby induce a reverse short channel effect. By canceling a short channel effect with the reverse short channel effect induced by the above-mentioned method, the short channel effect of the high withstanding voltage MOSFET may be suppressed.