In the related art, as an offset sheet-fed printing ink, an ultraviolet curable ink which is hardened by irradiating an ultraviolet ray is used. Further, recently, when printing is performed on a material which is hardly dried such as metal or plastic, other than paper, an ultraviolet curable ink is used to quickly dry the ink and increase productivity per hour. In order to harden the ultraviolet curable ink, generally, an ultraviolet ray irradiating device which irradiates an ultraviolet ray is used.
As the ultraviolet ray irradiating device, a lamp type irradiating device using a high pressure mercury lamp or a mercury xenon lamp as a light source has been known in the related art. For example, in Patent Document 1, a configuration in which light is irradiated on a workpiece conveyed by a conveyer belt by a lamp type irradiating device to harden the ultraviolet curable ink on a surface of the workpiece is disclosed.
Recently, due to demands for reducing power consumption, increasing a life span, and compacting a device size, an ultraviolet ray irradiating device which uses an LED as a light source, instead of a discharge lamp of the related art, has been developed. The ultraviolet ray irradiating device using the LED as a light source, for example, is disclosed in Patent Document 2 and a plurality of substrates on which a plurality of light emitting diodes (LEDs) is mounted is arranged to have a straight line shape to obtain a linear ultraviolet ray.