Abstract: A dielectric barrier discharge excimer lamp is employed as a lamp for emitting ultraviolet light, and this dielectric barrier discharge excimer lamp is turned on during a time period, which is shorter than a working period, for supplying a prescribed amount of ultraviolet energy to a substrate. Even if the working period changes, therefore, the dielectric barrier discharge excimer lamp is turned off when the ultraviolet energy to the substrate reaches the prescribed amount, so that the ultraviolet energy is maintained at a constant level and no excessive temperature rise takes place in the interior of an irradiator. Due to the employment of the dielectric barrier discharge excimer lamp, it is possible to stabilize lamp output within a short time after the lamp is turned on even if the lamp is repeatedly turned on and off, whereby the ovearll time for overall treatment steps is reduced.