To provide a method of manufacturing a semiconductor device, a method of manufacturing an active matrix substrate, and an electrooptic device in which in forming different type TFTs on the same substrate, a variation in the LDD length or offset length of TFT can be suppressed by a small number of steps. In the method of manufacturing an active matrix substrate, a patterning mask 554 used for forming gate electrodes 15 and 25 is left, and used in introducing a medium concentration of phosphorus ion to introduce impurities in self alignment with the patterning mask 554. Next, with the patterning mask 554 removed, low-concentration of phosphorus ion is introduced by using the gate electrodes 15 and 25 as a mask to form low-concentration source-drain regions 111, 121, 211 and 221 in self alignment with the gate electrodes 15 and 25. The LDD length of each of the regions is equal to the amount of side etching caused in patterning the gate electrodes 15 and 25.