An MRI system capable of acquiring a high-quality blood vessel image even during a measuring period other than at an injected contrast medium concentration peak by always picking up an image under optimum conditions while following up with an injected contrast medium concentration that changes every moment in the body. A flip angle is changed according to a contrast medium concentration b(t) as shown by a curve (102). During a period (Da) where a contrast medium concentration b(t) gradually increases, a flip angle (FA) increases according to a contrast medium concentration b(t); and during a period (Db) where a contrast medium concentration b(t) gradually decrease, a flip angle is gradually decreased. If a flip angle is controlled so as to follow up with a contrast medium concentration b(t) and reach an Ernst's angle at which a single intensity is at a maximum, a high-quality blood vessel image can be acquired even during a measuring period other than at an injected contrast medium concentration peak.