When the laser light is irradiated with high output in the manufacturing process for a semiconductor device, an attenuator is heated and cause a deformation due to the laser light scattered in the attenuator. As a result, the attenuation ratio of the attenuator fluctuates, and it is difficult to process the substrate with the homogeneous irradiation energy. It is a problem of the present invention to provide a laser irradiation apparatus, a method of irradiating laser light and a method of manufacturing a semiconductor device, which can perform the laser irradiation effectively and homogeneously. In the present invention, the thermal energy generated in an attenuator is absorbed by means of cooling in order to keep the temperature of the attenuator constant. By cooling the attenuator so as to prevent the change of the attenuation ratio, the function of the attenuator is protected. In addition, the energy fluctuation of the laser light irradiated on the substrate is also prevented. The attenuator includes an attenuator which separates a damper portion physically and cools the damper (refer to FIG. 4), and also includes an attenuator which cools the whole attenuator (refer to FIG. 7).