A laser annealing apparatus has a laser generating section for producing a laser beam, a splitter arranged to partially reflect and partially transmit the laser beam. The apparatus has a first energy converting section for measuring the energy value of the laser beam reflected from the splitter and outputting it into an electrical signal and has an energy control section for comparing the output signal of the first energy converting section to a reference value and automatically correcting the energy value by the difference between them. A process window may be positioned so the laser beam can pass through a built-in slit for annelaing a substrate. The apparatus further has a second energy converting section for measuring the energy value of the laser beam passing through the process window. In response to the output of the second energy converting section, the energy control section corrects the energy intensity of the laser beam.