It is intended to provide a method for evaluating an effect of controlling the activity of proline 4-hydroxyolase; a screening method with the use of the above evaluation method; and a method of screening a compound which is useful in treating or preventing fibrosis or rheumatoid arthritis. A method for detecting an effect of controlling the ubiquitination activity of proline 4-hydroxyolase alpha subunit (P4HA1) by synoviolin; and a method of screening a substance having an effect of controlling the ubiquitination activity of P4HA1 of synoviolin based on the above method. A compound found out by this screening is useful in treating or preventing diseases caused by disorders in the activity of proline 4-hydroxylase such as fibrosis and rheumatoid arthritis.