To provide a method of canceling an adverse event due to unbalanced diffusion in the longitudinal and lateral directions by a relatively simple way in a method of manufacturing a semiconductor device.

In manufacturing a lateral MOSFET 10, different masks are used when forming a P+-type diffusion region 12 and when forming an n+-type diffusion region 13. That is, when forming the P+-type diffusion region 12, the end of a mask is positioned at the B line, and when forming the n+-type diffusion region 13, the end of a mask is positioned at the A line. When diffusing p-type dopants which are doped in forming the P+-type diffusion region 12, a lateral diffusion distance is 85% or less of a longitudinal diffusion distance. A distance between the A line and the B line is one corresponding to the lateral diffusion distance.