PURPOSE: To ensure satisfactory finishing for printing on a corrugated board sheet without regard to changes in ambient temperature and humidity of an ink transfer mechanism.

CONSTITUTION: An ink transfer mechanism 48, equipped with an ink transfer roller 50 and a squeezing roller 52, is arranged over a plate cylinder 44. A metal support plate 64 is provided round the outer circumference in close vicinity to both of the rollers 50 and 52 so that the support plate can surround the rollers, extending over approximately entire length thereof in its longitudinal direction with a specified distance kept from the rollers. The support plate 64 is lined with a covering body 62 made of liquid-retention material. An ink supply device 54 that supplies ink to an ink storing part A is provided to a position above the ink transfer mechanism 48. By making the covering body 62 impregnated with a required amount of water, the temperature and the humidity in a space S formed by the covering body 62 and both the rollers 50 and 52 can be stabilized.