PROBLEM TO BE SOLVED: To provide a milk quality improvement agent or a mastitis preventive/improvement agent that contain a substance which is found to have an excellent milk quality improvement effect or an excellent mastitis preventive/improvement effect as an active ingredient.SOLUTION: A milk quality improvement agent for improving the quality of milk by being administered to dairy cattle contains an extract from plants of Macleaya as an active ingredient. And a mastitis preventive/improvement agent for dairy cattle contains an extract from plants of Macleaya as an active ingredient.