To restrain irradiation of a blanket cylinder with ultraviolet rays used for drying and to prevent occurrence of printing troubles.

An ultraviolet drying device 6 radiates the ultraviolet rays 7 from the point A of an impression cylinder 3 where it faces the blanket cylinder 2 to the downstream side in the direction of rotation of the impression cylinder 3. At an operation plate 13 of a detector 10, a first lighttight part 13a which prevents the ultraviolet rays 7 radiated to and reflected from the impression cylinder 3 from striking the blanket cylinder 2 is provided along the peripheral surface of the blanket cylinder 2. Besides, a second lighttight part 13b which shields a gap C between a safety cover 5 and the operation plate 13 is provided.