To provide a light radiation device capable of performing a light radiation process ensuring high in-plane uniformity of luminance in the light radiated surface of an object to be processed.

The light radiation device has a light source element composed of: a long lamp held by a housing at both ends in a lengthwise direction and having a rectangular cross-section perpendicular to the lengthwise direction; and a light transmitting, long, and cylindrical jacket pipe provided with both ends thereof in the lengthwise direction held by the housing while the lamp is inserted therein and defining a cooling air circulation passage inside. A supporter for directly supporting the lamp with respect to the jacket pipe at a partial position in the lengthwise direction is provided inside the jacket pipe. The supporter has a supporter body, made of an insulation material, which is in contact with the corners of the four ridges of the lamp, thereby sandwiching and holding the lamp. The supporter body has a jacket pipe contact part, which is in contact with the internal surface of the jacket pipe and has an elastic member.