When a laser beam is radiated on a semiconductor film under appropriate conditions, the semiconductor film can be crystallized into single crystal-like grains connected in a scanning direction of the laser beam (laser annealing). The most efficient laser annealing condition is studied. When a length of one side of a rectangular substrate on which a semiconductor film is formed is b, a scanning speed is V, and acceleration necessary to attain the scanning speed V of the laser beam relative to the substrate is g, and when V=(gb/5.477).sup.1/2 is satisfied, a time necessary for the laser annealing is made shortest. The acceleration g is made constant, however, when it is a function of time, a time-averaged value thereof can be used in place of the constant.