PURPOSE: To enable the surface being radiated to be flash-radiated in a short time evenly by disposing two or more flash discharge lamps within a fixed spacing near the front of a reflecting mirror and letting the area of projection arc satisfy the specific conditions.

CONSTITUTION: Flash discharge lamps 1W3 of semicircular form of different arc lengths are disposed in proximate in the order of sizes on the same plane and the optical axes X are so selected as to be perpendicular to said plane and pass the center of the circle. A plane reflecting mirror 4 of semicircular form is then disposed in proximity to the flash discharge lamps. In this case, the plane thereof becomes the selected projection surface and the projection area S1 of the arc of the flash discharge lamps is the total of the three of the sectional areas inclusive the arc center axis parallel to this plane. The minimum segment face S2 including the projection arc is the area encircled by the dotted line frame connecting the inside wall of the bulb of the discharge lamp 1 and the electrode of each flash discharge lamp at the shortest distance. By arraying a plurality of the flash discharge lamps, S1/S2 may be approached to 0.4 or more or close to 1. Thereby, the face light source of substantially semicircular form by the flash discharge lamp group may be obtained.