When a rectangular image having homogeneous intensity distribution is transferred by an imaging optical system, aberration adversely affects the homogeneity of the intensity distribution. The present invention provides a laser irradiation apparatus that can suppress the aberration due to the imaging optical system typified by a cylindrical lens, that can enlarge the square measure of the beam spot in which the intensity distribution is homogenous, and that can anneal the irradiated surface homogeneously efficiently. Moreover, the present invention provides a method for manufacturing a semiconductor device with the use of the laser irradiation apparatus. In the present invention, the divergence of the laser beam is suppressed and the size of the imaging optical system is miniaturized by using an off-axis lens array such as an off-axis cylindrical lens array. By the miniaturization, it is possible to reduce the cost, to facilitate the maintenance, and to suppress the aberration. By suppressing the aberration, the homogeneity of the intensity distribution of the beam spot can be improved.