A purpose of the present invention is to provide a novel diagnosis and risk evaluation method for cancer brain metastasis and a novel treatment and prevention method therefor by clarifying the mechanism of blood-brain barrier (BBB) disruption in brain metastasis. Another purpose of the present invention is to provide a novel drug delivery system to the brain. Still another purpose of the present invention is to provide a medicine, composition and kit to be used in these methods. Therefore, the present invention relates to a method for diagnosing brain metastasis or evaluating the risk of the occurrence of brain metastasis, said method comprising determining miR-181c level in a specimen. Further, the present invention relates to a method for treating or preventing brain metastasis, said method comprising inhibiting the expression or activity of miR-181c in a cancer patient. Furthermore, the present invention relates to a method for improving permeability through BBB with the use of miR-181c or PDPK1 and a method for enabling a desired active ingredient to pass through BBB and reach inside the brain.
