An improved device isolation method for a semiconductor device capable of independently and compatibly providing an isolation film in the interior of well and an isolation film between wells during a consistent process, so that latch-up characteristic can be improved even in a device requiring a design rule of below 0.5 .mu.m, which includes a first step which combines a second step which forms a device isolation film within a well and a third step which forms a device isolation film between wells, the second and third steps being compatible to each other during the same step.