To restrain the structural enlargement and complication of the whole printing machine and to easily and quickly perform operation of separating a printing cylinder from an ink supply device and returning them into an original externally-contacting state.

This offset printing machine includes an impression cylinder 5 and a blanket cylinder 6 in a circumscribed arrangement for holding a supply line of printed matter W; the printing cylinder 7 is circumscribed about the blanket cylinder; and attaching rollers 12 of the ink supply device 10 provided in external contact with the printing cylinder. The offset printing machine comprises a contacting/separating mechanism 15 for allowing the printing cylinder 7 to get into contact and separate with/from the attaching rollers 12 of the ink supply device 10, allowing the blanket cylinder 6 to get into contact and separate with/from the printing cylinder 7 and allowing the impression cylinder 5 to get into contact and separate with/from the blanket cylinder 6.