PURPOSE: To prepare the titled agent administrable continuously because of its low toxicity and useful for the prevention and remedy of especially organofibrosis of mammals, by using a specific 6-substituted-2,3-dimethoxy-5-methyl-1,4-benzoquinone as an essential component.

CONSTITUTION: The objective agent for suppressing the fibrosing of animal tissue is obtained by using the compound of formula (A is CH2 or CO; n is integer of 0W21; R is H or lower alkyl) as an active component. The compound has the activities to inhibit protocollagen.proline hydroxylase and suppress the biosynthesis of collagen, and is useful for the prevention and remedy of organofibrosis caused by the excessive accumulation of collagen, e.g. fibroid lung, hepatocirrhosis, nephrosclerosis, arteriosclerosis, scleroderma, myelofibrosis, chronic arthritis, etc. It is administered preferably at a dose of about 2W50mg per dey in 1W 3 times for adult in case of the prevention and remedy of hepatocirrhosis, arteriosclerosis, chronic arthritis, etc.